I don't own one but a couple friends do so I've had a go. Nice guns, the magazine can be a pain in the arse if you're prone (it's quite long) and mag pouches are less plentiful than say, MP5s.
Nobody seems to do a decent aftermarket nozzle for it so you're stuck with stock parts if it needs replacing, the stock isn't adjustable and some find it a little long. Nice guns though.
ImTriggerHappy will be along shortly to tell you that you should buy an ICS instead. Ignore him, they don't make a UMP and he's just got a bee in his bonnet.