Methodology:
I'm am going to write a mostly spoiler free review of this series as most of its faults can be detailed without given away any plot specifics (nothing you can't guess without having ever cracked the cover) aside from character names. Warning, this series is ecchi and carries all such related warnings. All of pronouns referring to Mizuki will be male because I'm lazy and don't feel like writing his/her and he/she etc. and I have no clue how the author intended for us to see the character.

Summary 3/10 :
In certain sporting communities the word potential is used as an insult. With that in mind, I can only say that Boku Girl had potential. It had the foundations of a generally good gender bent series; but due to a phenomenal lack of a structured story, character development, sense of urgency, and consequences for actions taken, it failed to leave any positive lasting impression and left me asking what the purpose of the manga was by the end.

Story 2/10:
This is the most baffling part of the series. Boku Girl by all measures appears to be a tale of ecchi romance, and perhaps part harem; but, it fails to develop believable organic romantic relationships between Mizuki and any of his love interests.

The story begins with the expected gender transformation in our protagonist and the author writes a good twenty or so genuinely enjoyable chapters that deal with how Mizuki must proceed if he wishes to hide it from the girl he's interested in and the school at large. Unfortunately, once his secret is revealed the plot becomes muddled and rapidly devolves into a standard American romantic comedy filled with misunderstandings and a general lack of honesty between the characters. This causes the story to stall out and prevents the characters from developing proper relationship with each other.

This is because there is almost no causality in the story. Various characters will make attempts to court Mizuki but their actions don't result in any consequences. For example, a great number of characters try to rape him but it never changes the way Mizuki feels about them. If they were creepy before hand, they still are, but if they were a close friend Mizuki doesn't show any signs of emotional turmoil that his trust has been betrayed. This may be written off as generally ecchi behavior because the story never recognizes it as anything other than normal, but this would have been a strong opportunity to force much needed character development and offered a chance to escape the character and plot stagnation that was occurring.

Art 6/10:
Credit is due for the art. I'll quickly let it be known that the scenery and the general quality of the art work was slightly above average and never left me wanting or confused. Furthermore, some of the characters like Mei and Mizuki has designs which fit the characters well.

Unfortunately, the same cannot be said for Yumeko, the female love interest. Her design was bland. It had no distinctive characteristics and made it very hard to see her as anything other than another pointless harem shipping.

While I can't complain about this specifically from a critical perspective, I thought the author should not have allowed the characters' designs to to so obviously explain their role in the story. E.g. random high school male is drawn like a creep so naturally he's going to hit on Mizuki and be a pest. Variety is the spice of life and although the author has plenty here, he isn't willing to break out of the mold and try anything daring and new. (Also, is it too much to ask that Thor be drawn with a bit more maturity?)

Characters 2/10:
Show don't tell: that's the best bit of advice for any character development that I can give. Boku Girl puts forth a rather impressive amount of effort to explain how the characters feel about each other. Unfortunately it never goes anywhere thanks to this series utter lack of causality. The author mainly develops the characters through introspection and acts of physical contact that will not be expounded upon properly.

I touched on this a bit in the story section, but the development of Mizuki and Takeru does progress nicely for the first twenty or so chapters. Unfortunately, across the board the author leaves the character arcs unfinished for essentially all of the characters. Minor characters get undue time in the spotlight for character development but then they turn out to be tertiary to the main plot and only distract from cementing the primary characters as their own discrete entities. In general, all of the characters are too passive and unwilling to express themselves constructively to the ones they love.

Mizuki: While initially his struggles to pass himself off as a boy define him as a character; once his secret is revealed, his defining characteristics become so difficult to see that you could say that the character by the end is a totally different character. This is very problematic because there is no natural progression from point A to B that you might expect from a gender bender. The character is so passive in accepting roles from others, that I'm not quite sure how the character will behave on his own. I would preferred to see Mizuki take a dominant role in relationships so that we could better have seen how his behavior changed due to his transformation and societal influences. Also, Mizuki's passivity is one of the main causes of roughly fourty chapters of filler material that leads nowhere. I suppose as a study for how social pressures affects humans it's not the worst effort I've seen, but I'm here to watch a story about how a kid deals with a gender transformation and how that affects his love life and not about how wearing a dress prevents a person from behaving assertively.

Yumeko: Honestly, I can't say very much about this character beyond that she's a generic female love interest. She's in enough of the story to develop her own character but the author didn't seem to want her to be anything else. (He even has another character make fun of how stereotypical she is)

Takeru: In general I'd say he was the most believable character. His initial relationship with Mizuki makes sense, but as the story progresses he begins to behave erratically. I can only presume that he was the intended romantic partner for Mizuki, but his relationship with him bizarrely flip flops between best friends and love interests in a manner that I can only describe as manic.

Loki: A generally fun character that sets the plot into action. Unfortunately, her antics, past the initial transformation, don't really contribute to the story and it feels weaker as a result of all of the page time she receives in the final 60 chapters.

Depiction of Men 3/10 | Women 3/10 :
Unpopular opinion, but I feel that it is necessary to look at how a gender bender portrays the difference between the sexes.

Men: Simply put, just because a man likes a women doesn't mean that he's going to violate her personal space and take any opportunity presented to molest her. Numerous times throughout this manga we see various men plan to and actively molest Mizuki for no other reason than he's cute and available. I understand that this is an ecchi piece but rape/incest leaves a bad taste in my mouth and it wasn't called for in this series. How is it that the specifically creepy character Yamada is more respectful of Mizuki than his childhood friend?

Women: Honestly speaking, it's a mixed bag. Some individuals like Loki and Mei stand on their own and I have no complaints about them. But for crying out loud, Yumeko is nothing more than a cardboard cutout that I've seen many times before whenever someone needs a generic female love interest for the man to fall for. Mizuki's female form is even worse. From the start of the manga Mizuki is objectified by BOTH genders as an object of attraction. Only Yumeko and Loki have an explicit interest in Mizuki for a reason other than his appearance.

Overall 3/10:
I can't say I would ever recommend this series to anyone as anything other than a textbook on how not to write a romance. I can't say that I wasn't enjoying the series while I was reading it. It presented enough comedy to keep me going, but that's probably the only major draw the manga has. To say I was disappointed by the ending is needless, because no ending presented would have satisfied me. The characters were simply too flat and dull to become invested in. That is quite the marvel in fact when you consider the length of the series. Manga such as Pluto have made me care for characters more in the course of two chapters than this manga did in the course of its entire length.