I'm 5'11, and I used to ride a 24" cruiser with about a 21.5 toptube, and even recommend that people just wanting to ride around and maybe race at the track buy a cruiser.
Now that I actually rode (and own) a decent 20" bike, I'd like to withdraw all of those statements and call myself an idiot for buying the cruiser. The smaller bike makes it so much simpler to do stuff, and it feels a lot more natural to do everything. I guess the bigger bikes might hold speed better, but you're pedaling almost the whole time if it's in a race anyway
(I guess just riding around town they'd be fine if you like the feel of a slightly smaller bike than most MTB's)