Three of my friends have MP5s. (two of these people are class III dealers and one is the wife of previously mentioned dealer) This is my first hand experience with other peoples guns. These are all dealer sample or transferable machineguns but believe it or not I have shot them on semi auto as well.
The MP5A2 is very accurate and reliable. I have seen a MP5 go all year without being cleaned with nothing but some occasional lubrication every 500 to 1000 rounds. This is not how I would treat one of my guns but sometimes people like to "torture test" their stuff.
The A3 sliding stock looks cool but is a little shorter and doesn't give a good cheek weld in my opinion. One of my MP5 owning buddies prefers the A3 stock for convenience of transportation and room in the safe.
Both of my friends use the K style vertical fore grip on their MP5s for subgun matches. They say it gives them better control when firing bursts. I like the feel of the regular straight forend for casual shooting.
I love the MP5K. At casual shooting distances (about 30 yards is our normal distance) it is just as accurate as a full size. I like the feel of the side fold stock on the K better than the A3 stock of the full size but it isn't as good as the A2. It is much more ammo finicky than the full size guns. It has to have hot loads to work 100%. 124gr NATO ball is what it wants, the full size guns work fine with most 115gr stuff.
I have given serious thought to getting a SBR MP5SD after shooting my friend's dealer sample. On semi the gun is simply amazing to shoot. On full auto it gets uncomfortibly hot pretty quick.
I have never shot a .40 MP5 nor have I shot the USC/UMP. I passed on a chance for a SBR USC/UMP clone because the mags were so expensive and hard to find.
AAAAAnnnnd.... gun pics! (because the other four letter word that starts with p is blocked by the language filter)