We are using SW2010 SP4 and it's working fine. I certainly would not want to be running SP0.
We are also running SEPM version 10.03.0843 and can't say that I find many errors. But we also don't use the workflow part of it so I can't talk to that side of it. I do know that this SP did fix some issues we were having.
We have a SPR for Search Performance Regression. Our searches are 4 times slower than what we see in EPDM 2008 SP4. They say is it vault specific but if you look in the knowledge base there is another SPR since SP3 with the same regression and others that were fixed in each version of 2010. Many people may not see the regression if they do not have a large database. We have around 2 million files.
Go back to your reseller and ask them for the specifics. The only real way to be sure is to set up a test system and test it yourself.
We are just getting started, so the our database is relatively small.
I would think that there are more things that are fixed in the latest service packs instead of problems being introduced.