Past Future is a gag comedy manga, if you went in looking for deep exploration or drama then you will sorely be disappointed, BUT if you like a little laugh (or maybe even specifically looking for this kind of genderbender comedy) then it's surely worth reading (it's pretty short after all).

Essentially the manga used Densha Otoko's famous plot device of "discussing problems on the BBS/Internet" thing as a central plot driver for Kako (the main protagonist) to do various shenanigans to get approval from his little sister (which is pretty genius to be honest), the gag is that his attempts obviously fails and he go to the BBS to report and continue the cycle.
There's just a bit of drama at the end to nicely tie up the story, and even in the climax the BBS is involved so it's not just an early gimmick or anything.

As you might've assumed from this format, it is slightly repetitive and the little sister acts unreasonably just to create tension, thus it is pretty lacking in the story department. Some (to be more precise, just two) side characters showed up to mix things up a bit, but in the end it's still just about the relationship of him and his little sister.

tl;dr: good if you want a quick laugh about a crossdressing dude, bad if you want good story.

- Scores:
Story: 3
Art: 4
Character: 6
Enjoyment: 7
Overall: 5