We prefer to belay directly off the anchor when multi-pitch climbing. The Mad Lock is the smoothest device for belaying a second rope while using a 10mm rope (a big plus). Belaying in autoblock mode on ropes thicker than 10mm with a Petzl Reverso 4 or Black Diamond ATC Guide is an elbow killer there is just too much friction. With the Mad Lock, the rope passed through 25-50 percent more smoothly than with either of those two devices when using a 10.2mm rope. It was a big enough difference that when head-to-head testing with a 10.2 on Goodrich Pinnacle in Yosemite, Chris Mac stopped using the ATC XP or Reverso after two pitches and just used the Mad Lock. The only devices that belays smoother off the anchor are the Trango Cinch and Petzl GriGri. However, those devices are are much heavier, more expensive, you can't belay two climbers with them and you will need a second belay device for rappelling. Additionally, the Mad Lock comes with a removable pin, which allows safer control of ropes down to 8.4mm. This pin can be added or removed with a dime or screwdriver.
When rappelling, the Mad Lock had an extra option for creating more friction by wrapping the rope around its hook. Usually we need either to rub the rope against our leg or run the rope through a biner on the leg loop to create more friction. This hook feature made adding more friction very convenient.
This device is light. If you carry both pins in it is 3.2 oz. but if you take the pin out it is only 2.8 oz, which ties it with the Reverso 3 as lightest auto-blocking belay device. Finally, the hook on the device works great as a bottle opener. A nice feature at the end of the day.
The hook/bottle opener sometimes catches when paying rope out fast to a leader. This doesn't happen every time, but because you are never sure when it will happen it makes the device a little nerve-racking if you are belaying someone on lead where a very tight belay is necessary. This wasn't a huge problem it can be minimized in a critical belay situation by paying close attention to how the device is oriented.
Part of what makes this device so versatile is the removable pin that accommodates different rope sizes. The problem is that this pin is easy to lose and not practical to switch out during a climb. You can solve the problem in a couple of ways. Either commit to thick or thin ropes at the beginning of the day and use the pin accordingly, and/or carefully keep track of the pin in a zippered chalk bag or elsewhere.
Finally, while you can add friction by wrapping the rope around the hook, if you don't do this there is less friction than is common with other devices.
Bottom line: our dislikes can be dealt with if you are willing to change your belay habits a little. Keep track of the pin, wrap the rope around the hook when rapping and pay attention when belaying to avoid a possible catch in the rope.
This is an ideal all-around device, especially if you want to keep track of only one device or do a lot of multi-pitch climbing.
The Mad Lock has the lowest price tag of any auto-blocking belay device on the market by a big margin.