Stay away!
BTC Guild is a merged mining pool (like most pools). You earn Bitcoins _AND_ Namecoins simultaneously. You get just as much BTC as you would at any other 5% PPS fee pool. Merged mining takes a valid Bitcoin hash and applies it to Namecoin as well. No speed is lost. No coins are lost.
The front page explains it quite clearly.
Oh sorry, I assumed power was wasted, can I sell NameCoins too ? I am comning off Deepbit so was a bit confused.
You can exchange namecoins on a namecoin exchange like Bitparking (https://exchange.bitparking.com/main). They're not worth much at the moment (~0.003 BTC per NMC), though sometimes they get a spike in trade volume which temporarily makes Namecoins a decent bonus (1%+).