I recently had to opportunity to test and review the Mongoose M 3.5b Gimbal Tripod Head for super telephoto lenses.
The first thing I have to say is that I am very impressed with this compact and lightweight gimbal head. Very lightweight and packs easily. Craftsmanship is superb, and you can tell the folks at 4th Generation Designs really paid close attention to detail.
Check out this short video review.