I thought I'd start a general topic on this because I'm researching to figure out what I need, after I realized the Apple TV wouldn't cut it. Here's what I've found out so far.
Apple TV - Great if you buy all your video through iTunes, if you don't, you're out of luck until it's hacked. HDMI output, can stream as well as hold content locally, wired and wireless ethernet. Only really plays AAC, MP3 and H264, but will probably have a great interface as per usual with Apple.
Tvix/Mvix/Mediagate - These are all based on the same chipset so they have the same basic interface. They'll play any media format under the sun, however the interface almost universally sucks. Plus, hard to find in the US.
Netgear - Netgear has a media center out which is not supposed to be bad at all, and they have a new version coming out with H264 and HDMI support soon. Oddly enough it won't play OGG music, not sure why. Still keeping my eye on this one, the interface is not suposed to be bad on the EV700, which is the current model.
D-Link - Their media centers play lots of content, but overall they're supposed to be buggy and unstable. Cheap though.
Does anybody else have any experiences with this stuff? I'm still sorta stuck on this because almost every media player has some major faults or another. I'm still hoping someone hacks the Apple TV but I won't hold my breath.