1) 1.2.2 on 6 to 12 EC2 m1.xlarge
2) Quorum R&W . Almost no deletes (just some TTL)
4) On each node once a week (rolling repairs using crontab)
5) The only behavior that is quite odd or unexplained to me is why a repair doesn't fix a counter mismatch between 2 nodes. I mean when I read my counters with a CL.One I have inconsistency (the counter value may change anytime I read it, depending, I guess, on what node I read from. Reading with CL.Quorum fixes this bug, but the data is still wrong on some nodes. About performance, it's quite expensive to run a repair but doing it in a low charge period and in a rolling fashion works quite well and has no impact on the service.
Hope this will help somehow. Let me know if you need more information.