Yes, I've just acquired one having done a straight swap with Moomike for a ball head. The only reason he parted with his was he sold his long lens.
I have a Dietmar Nill gimbal head also and am very happy with it, but it costs 5 times the price, so I was always going to be critical of the Manfrotto.
Having used the 393 a couple of times though, I have to say I'm seriously impressed with it. Where the Nill is cast aluminium the Manfrotto is simply bent to shape, but it's a very nice sturdy bit of kit at a great price and I wouldn't hesitate to recommend it.
Plus points
It comes with a long plate to mount to your lens which should give you enough slide adjustment to balance the longest lens with or without converters. The mount also has a very nice safety device built in, so it's impossible for your lens to slide out of the mount either backwards or forwards without pressing a safety release catch. The extra height of the 'U' bracket gives a huge range of movement up and down which would be an advantage for tracking flight shots, and the torque adjustment is very nice - the lens stays where you put it regardless of the angle.
Minus points
Instead of opting for the universal Arca Swiss type mount, Manfrotto have gone for their own design, but this isn't a big issue if you only intend to use the head with one lens, and I dunno if Manfrotto do different length plates for other lenses. I'd be very surprised though if Manfrotto or someone else doen't do a conversion shoe for Arca plates - the mount is wide enough. As I say, no biggy anyway if you only intend to use one lens.
Unlike the Nill head where you can just drop the lens foot on from above and then tighten the clamp, with the 393 you have to slide the lens into the shoe from behind. No real biggy again except that mounting these big lenses on a tripod and unmounting them is when you're most at risk of dropping them, but you soon learn to develop good safe techniques.
It's bigger and heavier than the Nill, but it wouldn't stop me getting one.
Frankly it's a bargain compared to the alternatives, just get one.
