I don't have experience with other trackers, but I can assure you the Astrotrac is an absolute gem. For months I hadn't realised I had the tangent arm the wrong way around (I blame the manual!) and even then I was able to get several minutes of sharp stars using wide focal lengths. After correctly setting up and polar aligning the astrotrac I've been able to achieve 5 minutes of perfect tracking using a Canon 200mm lens. I probably could have pushed it even further but didn't try as I was happy with that.
I'm a member of the Astrotrac yahoo forums and have seen people do some crazy things with their astrotracs.. like mounting a C5 SCT on it, 4" refractors, and even a 152mm Takahashi refractor!!! (
http://cosmicbug.files.wordpress.com...t320-glee1.jpg).
Personally I think the Canon 400mm f5.6 is perfect for the astrotrac. Decent focal length for dso imaging and with good polar alignment you don't need to autoguide. My 2c.