good as a harness, but...
I purchased four inflatables about ten years ago, and use them whenever I feel it's necessary to tether myself to the boat. Usually only offshore when I've run the jacklines. Of course, you have to buy the ones with the D-rings for them to be of any use as a harness.
Whenever I'm bay sailing, which is most of my sailing, I prefer to use my high-end kayak vest. It's actually more comfy than the inflatable, especially if you're going to be sailing more than a couple of hours. I'm noticing that a lot of local racers have moved in the same direction over the course of the past five years.