If you are tied into the Apple ecosystem, the AppleTV makes a lot of sense. And if you aren't a member of Netflix, it makes sense too. You can rent a lot of stuff (movies, tv shows) via AppleTV and you don't have to have Netflix. If you have Netflix, the interface is really very nice.
I have an AppleTV and the interface is really quite simple and elegant. I switch the TV to the AppleTV input and I can use either the remote it came with or my iPod or iPhone as a remote. I like it. There are a number of outside subscriptions available - I think to MLB (don't quote me on that) NHL, NBA.
Roku is cheaper (I think there is a $49 unit) and features the ability to get into a lot of different online channels. Most of the content that is available via the AppleTV (except iTunes stuff) is available on the Roku box. One website that I read regularly reviews the Roku stuff - zatznotfunny.com - and you might want to check it out.
On certain cable systems you can view HBO content (with a subscription) on Roku - I don't believe Comcast allows it based on what I've read.
Hope that helps.