I stayed at the Schloss Fuschl in August using resort nights from the promotion Starwood was running earlier in the year, and don't think I'd return.
The resort itself is fairly nice, though not worth anywhere close to the 550 Euro per night price tag. More than anything else the crowd was very stuffy, so don't expect to walk around the place in anything short of slacks and a collared shirt without being looked at strange.
I was also denied an upgrade at first because the agent claimed upgrades were at the discretion of the hotel because I was on a "free" stay. Tweeting @SPGInsider got that solved, though it certainly doesn't reflect positively on the hotel.
For anyone that's interested I wrote a full review of the hotel with pictures on my blog, which can be found
here.