Heads up to all on a sale I've been hoping would happen. I just read about it on a cooking site. I've been hoping to get one but the price was just a tad too high. Now I haven't an excuse.
I use one in the kitchen--I think I paid $79 a few years ago, so $69 is a good price for these. It pretty much works as advertised. I've used it for candy-making. I might pick one up for the darkroom--somewhat obviously, kitchen and darkroom gear is not interchangeable! I only wish they made one unit that could switch between deg. F and C.
I actually have a meat thermometer as Kirk says - don't remember the brand - which I bought for about $30. It does the job very well. I would not spend double the money for such thing.
Speed and accuracy, mainly. Greater range, though that's usually not a big issue. I've tried less expensive thermometers, and none of them respond as fast as the Thermapen.
The cheaper ones are plenty accurate enough. I have used them for the very temperature sensitive C-41 and E-6 processes with good results. I've tested any number of these things against a Paterson Color Thermometer. They all come withing a few fractions of a degree against it as a standard. As for speed, it's just not that important a consideration for my needs.
For what it's worth, I ordered two. One for darkroom and one for kitchen. They just came so I haven't had time to break them, so it will be a while to have a sense of durability. Regardless, they're fast reacting. I'm pleased so far.