Yeah, work on them quite a bit, mostly replacing tonearms with broken headshell connectors or replacing the RCA/grounds. If you can solder and understand the technical manual, then you should be fine - if you aren't confident in those areas, take it to a service professional. Most of the service places will be $$$ though, plus the $$$ of shipping - if you have something wrong with MK2's it is often more cost effective to replace them then repair them. Roughest part is rigging it so it is suspended upside down without damaging the tonearm, powerswitch, light, or pitch adjuster. Best pieces of advice I can give:
1) See if you can find a broken one for spare parts.
2) Double check those solder points!
3) If you aren't confident, don't bother even opening it up.
4) If you do open it up, go ahead and upgrade the RCA's to Monster cables or similar quality, and replace the ground wire while you are at it. Also, go ahead and spray out the dust with canned air.
5) Work on it somewhere where you can plug it up and put a needle on it while it is still disassembled to test the signal by touching the stylus - big time saver!