I have this same router. I got it last week at BestBuy as well. MUCH better than my 'high end' WRT400N from Linksys that had trouble providing me more than 12mbit over wifi on ANY device. I played with DD-WRT, the DD-WRT "Kong" mod, Tomato, etc. I ended up flashing back to the stock firmware. I was going to use the NAS functions, but it just wasn't fast enough. Ended up buying a separate WD MyBook Live, and leaving the USB port unused. The community for the router is GREAT, however.
Tomato worked OK
DD-WRT was meh, like DD-WRT is wont to be.
Stock firmware did everything I wanted it to do, and supports NTFS file systems on the USB port should I ever want to do that.