I did this ride at the weekend, it's quite a well known loop. It's in the Scottish Wild Trail book as well, and is recommended as one of the best bike rides in the whole of Scotland. It was a good big day out on the bike, but it didn't leave me buzzing like other routes I've done around there.
The scenery was amazing, with some lovely sections especially at the Falls of Tarf. There were also some great "big mountain" views. I guess about 40km out of 55km is on land rover track though, and the singletrack sections weren't the best. It was windy day though and the most exposed sections were all into a big head wind which didn't help. I would do it again, although I'd try it anti-clockwise and on a calm day. It's a good long route, plenty of exercise but I can think of two other big routes from Blair Atholl that have more variety and I've enjoyed more.
I'd be interested to hear what other people think about it. It's insipired me to try and ride all the way along Glen Tilt to Braemar though which would be cool.