In reply to ERU:
I think it is the best thing there is for half-ropes, where you often want to be paying one out and taking one in simultaneously. There is nothing on the market that handles as well as the Alpine Up for those situations, and you get solid assisted locking, even on the thinner half-ropes, which sometimes provide a very nasty surprise for belayers faced with stopping big falls on one strand using a tube-style device. The Alpine Up is, in particular, better for half ropes than the Mammut Smart Alpine, which is its only real competitor if you want assisted locking.
The only drawback, which the Mammut device shares, is rappelling. At least with my 8.5's, there's rather too much friction if used in assisted locking mode with 60m of rope hanging. CT has devised a work-around, which uses an additional carabiner to relieve rope weight on the device. I haven't yet had a chance to test that technique so can't comment on whether it makes things work better.
It's too bad if it doesn't, because rappelling in assisted locking mode means the rappel is automatically locked off if you let go. I've been using the UP's non-locking mode for rappelling, which provides about the same friction you'd expect from a Reverso or XP. Of course, if you are a believer in rappel back-ups, then you'd use one in this configuration.
I'm not a big fan of guide-mode belaying off the anchor, but resort to it at times when my back is sore. The UP works fine in this mode as far as I can tell, but I don't do it that often with any of my devices so can't really make good comparisons.
Someone commented that it took a while to learn how to thread the different options. I didn't find this to be true at all, and in any case there are clear threading diagrams on the device.
The downsides are price, size, and weight. In all three categories, it is comparable to a Grigri. The Smart costs half as much and weighs a lot less, but in terms of price remember you'd also need two Smarts to cover the range of a single UP.
CT says the UP can be used on half and twin ropes from 7.7mm to 9mm and on single ropes from 8.9mm to 10.5mm. Most of my experience has been on 8.5mm half ropes. I've belayed with the device on a 10mm single rope with no issues, but I have my doubts about decent handling with a rope as big as 10.5mm.