While I'd love to recommend a YC startup, Coinbase charges much more in fees than MtGox or CampBX, for example. Yeah, they provide more features and a cleaner UI, but if you want to just buy some bitcoins, stick with lower fees from established players.

Whoever will be the first to create a shopping platform, as Coinbase plans to, will very likely get a nice boost and probably become the #1 player.