It's a nice upgrade to have if you have some extra cash and have already gotten most of the other equipment you want.
I've been using one since last spring. The short version is that at first I didn't really like it much better than my standard micropulley and regretted shelling out for it. But after awhile I got kind of addicted to it. It's well made, rounded edges all over, and actually tends smoother than my other non-fixed cheek pulleys. My buddy decided he likes it well enough and just ordered one.
One problem as is mentioned above is it works best with a spliced eye. If your prusik is clipped in the bottom hole and your rope end is clipped in the top hole like I think they show on their website, a knotted end will be right at the top part of your VT. But since I'm not partial to a spliced eye on my climbing line, I just kept trying different things. I've settled on simply using the middle hole. First it pulls my knot a little lower and the VT rides above it. But since the holes are offset, it also seems to tip the whole thing forward a bit and put a little space between the two lines. Haven't had any problems with interference with this arrangement.