Opinions on the MP5 vary considerably.
Personally I think it's a superb SMG. It's very accurate, highly controllable, and they tend to be very reliable. The lack of a bolt hold-open can be kind of annoying, and the long trigger pull isn't ideal for a long gun. The safety/selector lever isn't as ergonomic as it looks, leading a lot of SOP's that involve running the gun off safe once you've put it to fire.
I've heard others who have a lot more time carrying MP5's than I do (generally in SOF units) who curse the MP5 as useless for anything but a diveweight.
As SMG's go, I prefer the Colt's AR based ergonomics and bolt hold open...but the Colt is more difficult to control in full auto, most require mounting optics on the top of a carry handle, and the mags are a pain to load.