I've been in the process of rebuying the Companions off of ebay and here's a bit of advice for anyone buying any RM supplements off of ebay - be PATIENT. It varies so much, set your maximum bid and stick to it. Set up email notification to notify you of new listings. Last week a Companion I sold for >$50, I bought one yesterday for $10, while at the same time RMC VII sold for $40.
I agree Companions I and II were the best ones, III was good too. However, there really is some fantastic rules, or at least seeds of ideas of rules in all the rest of the Companions. You can tell many aren't fully fledged out, or playtested much and in some cases may just have been a off the cuff idea they put in as filler. But there really are some great ideas, good inspiration if you are looking for ways to change things in your own game.