Excellent belay device. Consider this: even if not used for belaying two, one can use the dedicated loop to directly attach the device to a fixed anchor, so in case of a fall, you will not be dragged.... The ATC XP can't do this; however the Petzl Reverso is also capable of such functionality

Intro level harness; would not recommend for lead, or for any route that has an increased potential for falls and need for (self)rescue.

I bought this unit as soon as it became available in the US -- 2+ years ago. It has been used for mountaineering, skiing, training for , and during a marathon (for hear rate zones, etc). I have been very happy with the heart rate monitoring and the the software that comes with the watch. In fact, the software (in ease of use, data processing, and editing) beats the brand-new Garmin 305. The IR function to "beam" the files to your PC is sluggish, and often fails if the ambient lighting is too bright...
The altimeter is fairly good, but often needs re-calibration, while the temperature function is essentially USELESS. It is never accurate, not even close, unless you take the watch off and leave it untouched for 5+ min.
The worst function is the compass. It failed completely after 1 year of having the watch, at which time I got a new unit from Polar (under warranty) as they could not fix whatever the problem was. Now, about two years later I have the same problem... I did not drop the watch ever, did not submerge it, or did anything "extreme" to it.
In summary, this watch is great for heart rate monitoring/zone training, for its altimeter/pressure functions. The other perks don't work as reliably, if at all.