i know of mediaportal, and you certainly get what you pay for.
the interface is very nice, the quality is fine, it has customizeable controller maps, which are all nice features, but it's anything but stable.
You can count on having to get up and alt-control-delete and hard-stop mediaportal on a regular basis.
In fact, i could give you a LIST of MCE alternatives, and their respective downfalls compared to the original.
I've tried every one i could find. Nero, pinnacle media center, jriver, tvedia, sagetv, windvd, and of course mediaportal.