How do I find the hash "market share" of P2Pool? P2Pool is a peer to peer mining pool, offering some advantages over centralized pools:


There is no central node to take down (assuming the pool is indeed fully decentralized). Pools have suffered from DDOS attacks in the past.
An open source P2P pool should theoretically be able to operate with zero fees.


How can I find out how many people are using P2Pool?
In concept, P2Pool is much stronger than centralized pools. Unfortunately, most centralized pools still hold a high hash-rate.

You can see the global hash rate, and its distribution at http://bitcoinwatch.com/ 

My opinion is that miners are used to using slush, deep bit, and btcguild - and are either unaware of the incentives of P2Pool, or don't care.