I have one, and I also have Quarq. But I do race and follow a training plan. No coach or plans for one though, I enjoy doing this on my own.

Between the two the PowerTap is easier to use and been more durable, although when working neither is difficult. In favor of the PT is that it's readings seem more stable in regards to temperature change and it has worked perfectly for 3 seasons. My quarq developed a battery eating issue and had to be replaced in less than a year. The quarq stays on my race bike so I can change the wheels out. The PT mostly stays one bike I use for training, long rides and winter riding.

If I didn't race but still wanted power I would stay with the PT. They are cheaper, reliable and give consistent data.

As for its usefulness in that capacity, I don't really know. They are most useful for comparing and maintaining output when interval training. If you aren't ever going to do that I think the novelty may wear off.