From personal experience, I have the EX470 model, which I paid $320 for back in December (it has on sale at hpshopping). I love my MediaSmart Server. It serves as my central iTunes server and media repository. It's packed full of music, photos and movies.
I do also have Windows PCs, which the MediaSmart makes backing up extremely easily. I'm looking forward to the upcoming free software update that will add Time Machine backup capability. Word in the Windows Home Server community is that Microsoft is going to release another big update to WHS sometime this summer, which will be free and add additional features.