I bought one a couple of weeks ago. I currently use it will a 300 f2.8, 70-200 f2.8 and now a 16-35 f4. I did use it with a 17-55 f2.8 in the Lake District and Peak District last week.
So far I've found it to be very good and useful, being able to use all my lenses on one head is good. I first bought a ballhead and liked it, the only problem was tensioning the ballhead enough or it could fall sideways when letting to. I then moved onto a gimbal head which I really liked too, the only problem I had was getting low as possible and being level. It meant playing with the tripod legs until the gimbal is straight otherwise rotation is not straight. The uniqball is not a replacement for gimbal as it doesn't function exactly the same. It can't be left at crazy angles without the camera/lens falling forwards/backwards but it does balance if left straight.
To use it you basically pull the tripod legs out, level the red ball using the spirit bubble and then loosen the tension on the black ball for the required amount of movement you want. Levelling the red ball does take a little practice, I'm sure I will get faster the more I use it. The black ball will move left/right and up/down, but it won't fall sideways like a normal ballhead.
Where I find it helpful is landscapes as once the red ball is levelled I can worry about composition without worrying about not getting it straight. The main benefits to me are the straight images once it's setup properly, it only weighs 500g (I have the small one) and I wanted one head that could be used off wildlife and landscapes rather than swapping heads. It has made carrying a tripod easier, I have a manfrotto 055cx4 that only weighs 2kg with this head, the gimbal head was 1kg heavier and much bigger.
The real test will be a few days on skomer later in July. It's helped cut down the weight I will be carrying which will be welcome. Before buying it I was looking for a smaller, lighter head that used acra Swiss and could be used for landscapes and wildlife. I was looking at a ballhead and sidekick in the classifieds until I saw the uniqball, I thought I would give it a go. I probably will buy another gimbal in the future, but one that uses acra Swiss plates this time.
Edit: One thing I forgot to say for landscapes it came with the x adaptor. I have since found found I will also need an L plate to take portrait images. The direction the acra Swiss plate faces is important to allow the uniqball to operate properly. The plates also need to be attached to the lens/camera straight otherwise it will cause issues with the uniqball not being exactly straight.
