(Because everyone in here should have seen it, I'll be going into Spoiler Territory - so if you haven't, don't read below this...)
I was pleasantly surprised with it. I felt Schumacher had lost his touch (His early Lost Boys was fun trash, his Batmans were horrible, stinking, rotten garbage) but Phone Booth entertained me throughout.
It was fast paced, and ended at the refreshing 80-some minute mark (Pieces of April was another recent quick-paced movie - note to Hollywood: Not everything needs to be 2+ hours). The script was smartly written, as everything develops organically and realistically. Nobody acts stupid just to serve a plot-point. Everyone acts believable. Plus, Forest Whitaker is also in it, so it gets added points as he's the most underrated actor in the history of Film itself (probably...).
The main reason I liked it, though: the Bad Guy gets away at the end, and in most mainstream Hollywood movies, the only way that would happen is if the "Bad Guy" is converted, or changes his ways, or turns out to be alright, or something else to make him less bad. But this guy was just a sick monster with no redeemable qualities whatsoever, and he still gets away scot free. Nice touch.
So overall, three stars outta five. You gets your moneys worth, and it does everything it sets out to do.