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I can't remember what the brand was that we got, but my girlfriend's parents got one for their computer. They are foster parents and have also adopted four kids (all with medical problems). One of the kids (5 year old boy) has cerebral palsy and we got it for him in hopes that he'd be able to use it easier than a standard mouse or the large trackball mouse we tried (a kensington that had incredibly bad movement).
Anyway, we got one covered through the health money allocated to him and it works alright. It was actually a real pain to set up (the drivers that came with it were worthless and the ones on their site had issues too). I wish I could remember the brand, but they're in a different city.
You have to calibrate them each time you attach them to a display which involves running a program and touching the screen in a few places according to something displayed on the screen behind it.
I tried calibrating several times on the one we got but still it was pretty hard to get really precise movement on it. If it's something simple like hitting big icons, then it will work fine, but for something like highlighting text in a document or moving files around to different folders, it's not going to work nearly as well as a mouse would.
Maybe I just wasn't used to it (only used it for a few minutes), but it didn't feel natural and everything took a really long time to get done with it. Oh, and my arm also got tired pretty quickly holding it up in the air pointing at the screen and moving it around rather than having my wrist rest on a mousepad / table.
I don't know how much of this info is true for all touchscreens or just the one we got, but I think you'd have to really want the touchscreen functionality for these to be a good idea.
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