In reply to PaulHermes:
If you are going to belay with half ropes a lot, I think the Alpine Up is the best assisted locking device. It in size and weight it is about the same as a Gri Gri 2, which many people have deemed not too big to carry on multipitch routes (such as El Cap...).
Competent half-rope belaying has repeated instances when the belayer should be paying out one strand and taking in the other. In these situations, having to commit the brake hand to levering out the device makes the handling of both the Megajul and the Smart Alpine distinctly inferior to the Alpine Up, which leaves both hands free for rope handling. One of the things I like about having both hands free and assisted braking is that I can safely and effectively use my brake hand palm-up rather than palm-down as you do with a tube. I find managing two ropes without locking the device easier with the palm-up position. This is not an option with the Megajul or Smart, which requires the brake hand to be palm down in order to operate their release mechanisms.
It is possible to lock up the UP accidentally, although I find this happens quite rarely. The cure is to place the heels of both hands against the device and shove it away from the body.
I've tried the Smart Alpine and the Megajul and have several years of experience with the Alpine Up. (For a tube device my current favorite is the DMM Pivot.) Most of my experience is with 8.5mm Mammut Genesis half ropes. With those ropes, lowering is not much of a problem. I've also used the UP, just for test purposes, on fairly new 10mm gym ropes and had no lowering trouble with them. But if the ropes are any bigger than 10mm and are fuzzy, the UP is not going to perform well and something else should be chosen. For top rope situations with the 8.5's, I use the ``non-locking'' configuration most of the time, which has no lowering issues.
Top-rope set-ups have lots of friction and the non-locking UP configuration is fine. But I don't think that configuration supplies adequate friction for trad belaying of big falls, so using it for more dynamic belays on poor gear strikes me as a potentially bad idea---the belay may be a lot more dynamic than anyone would want.
Similar considerations apply to rappelling. The UP is fine as an auto-backup rap device with 8.5's. On absolutely free-hanging rappels with full 60m rope weight hanging on the device, I find I have to manually feed rope into the device for the first twenty feet or so, after which I can slide down in the ordinary way. This doesn't happen for me on shorter rappels or ones with foot contact. Climbing Technology gives a special trick, using an extra carabiner, for rappels with a lot of rope weight. I've tried this and find that the system to twists my ropes badly, and so I prefer the twenty feet of manual feeding when those situations arise. By the way, my subjective impression is that the UP twists ropes more than a tube device when used in locking mode even without the extra carabiner, but the twisting isn't as bad as with the extra carabiner and has been manageable with my ropes.
The locking feature of the UP makes it especially easy to ascend rappel lines. After taking some photos on rappel, I've been able to ascend twenty feet or so on just less than vertical rock by simply walking up the rock while hauling in the rappel strands.
Unlocking the UP at the base of a rappel can be annoying, because you have to reduce rope tension in order to unlock the device. If the stance allows it, I'll rappel to a kneeling position and then stand up to release the device. At hanging stations, I'll clip in with my tether and rappel until it is fully weighted. I can then push an little bit of slack into the device and unlock it.
All of these observation apply to using the UP with 8.5mm half ropes. I think rappels get a lot worse with ropes in the 10mm range.
The performance of both the Megajul and Alpine Smart depends significantly on what carabiner you use with them. This may well be true with the UP, but CT has eliminated the problem by forcing you to buy their carabiner with the device; a good idea in light of what I've read about the performance variations of the other gadgets.
The UP is not particularly good at belaying one or two seconds directly off the belay anchor---the pull-through resistance seems to me to be subjectively greater than many of the other devices. If this ability is very important, you probably want a DMM Pivot or, in the assisted braking category, the Alpine Smart.