I second everything ARG and awillis said. I researched the heck out of the various LPM options out there when I was in the market for one, and especially for the money, the Ophir is the way to go IMHO. Two batteries and a multimeter are all you need in addition to the head, and the response time can't be beat.
If you're on a budget, the Bluefan option is reasonable (at about $125), but for that much, I decided to spend the extra $75 and get something that's worth many hundreds of dollars more (as they really are on the order of $700 new). Also, the upgradeability of the Ophir option is kinda neat, if you want to make it fancy over time with datalogging, etc, but the 2 batteries + multimeter route is pretty slick too, and works perfectly fine.
-Andy