Belaying the leader or bringing up a second in traditional manner is as smooth and easy as any other device. Abseiling is equally simple and the extra metal on the Reverso allows heat to dissipate faster than on standard belay/abseil devices. In its auto-locking mode, you can bring up one or two seconds from a direct belay. In most circumstances this will be safer and easier than conventional belaying, especially when two seconds are climbing at once. Obviously you will want to be sure of your belay anchors before using the device in this way. Lastly, it can be used to ascend the rope.
The downsides? Well there aren't any major ones really. Some concern has been expressed as to the safety of the device in auto-lock mode when using very thin ropes. The Petzl website goes into detail about this. Testers have found that it seems fine with ropes as thin as the 8mm Mammut Phoenix, but you would be well advised to make your own judgements on this. One other point to bear in mind when using the device in auto-locking mode is that if a second falls into space (under an overhang for instance), then letting the rope out to lower them will become more difficult. Common sense, forward thinking and reading the instructions should avoid any problems.
For long routes, summer, winter or alpine and especially if climbing as a three (surprisingly efficient if done properly), the Reverso is highly recommended.