You make very good points - I was loath to switch from Media Center but I got sick and tired of MS doing nothing to support Canada, and taking over a year to fix the CGMS-A bug that affected Canadians. The last straw was when TV Pack came out last year and did not support H.264 and it broke Canadian OTA. I didn't think it likely that MS would support the HD-PVR so that is why I switched to Sage.
I don't mind the UI - I use SageMC, and with Fanart you can get it looking very good. I find Sage (the company) to be very supportive as they add new features on a very ruglar basis. They do significant upgrades about once a quarter - they recently added the ability to play back ripped BluRay DVDs. Plus the UI change was minimal for the family as I was switching from XP MCE and SageMC is (essentially) a clone of XP MCE.
My HD-PVRs have been very stable and my Sage setup is also extremely stable - I go weeks without rebooting my WHS/Sage server. The one thing I haven't yet tried is switching to digital audio with the HD-PVR, apparently that can cause problems.
It would take something pretty extreme to switch back to MC, especially since extenders for MC seem to be dying. The Xbox360 is ok for one or two rooms but I don't think it is all that suitable where you have four extenders, like I do, as it is overkill in most of the rooms, not to mention big, noisy and slower to start up.