I'd recommend getting the mega city undercover trade (or even better the new low life book) and starting from the beginning. Mainly because a) there's some really great artwork in those early stories and b) it's probably best to get up to speed with all the supporting characters. In any case they're all worth a read - istr enjoying them, with the possible exception of the one set at a gig/festival, which is a bit of a stinker.
It is quite a different strip in it's early days, though. Very downbeat and gritty.