I started mining about a month ago. I started with Eligius because there was no account creation needed, then evaluated BitMinter for merged mining and finally settled on p2pool.

Eligius is easy, transparent, and only keeps transaction fees. (~1% by my gorilla math) I use them as my failover pool. I was about to try BitMinter but they announced that they were increasing their mandatory fees (to ~1%, but they share transaction fees) so I found p2pool. I now have > 100% pool efficiency so I get a bit of a boost there and have the option of merge mining. Since this is becoming a bit of a hobby, p2pool lets me tinker, have more control, and contributes to decentralizing the network.