Thinking of buying a bike for the summer and was wondering if anyone out there knows a thing or two about em?

Only had a few bikes in my days, a couple BMX's and a mongoose, all one speed for jumping and messing around mostly. Also had a POS bike from the local store that had absolutely crap derailers and brakes, that bike didn't last long.

So I'm curious if anyone knows a bit about em? I'm probably not gonna destroy the bike so I'm looking for something fairly good, probably not quite up to the standards of a mongoose but who knows. Anything in particular I should look for when looking at the local stores? Shimano derailers of some sort I'm sure which seem to be the industry standard but which ones suck and which ones are gonna last a while? And I'm curious if anyone has any opinions on the two types of shifters available?

Thoughts, comments...?