I've seen a few of these on egay. My manfrotto has a ball-head which makes it a PITA to pan shots smoothly whilst keeping the camera perfectly horizontal.

I've thought about buying one.

http://www.ebay.co.uk/itm/Camera-Hotsho ... 27c80a2a95
http://www.ebay.co.uk/itm/Camera-Hot-Sh ... 1c23748782

Has anyone used one? Found it useful?

I think Alex has one.
My tripod head (also a ball head) has a horizontal lock which allows for smoothly panning the camera.

Mark

Mine is this one: http://www.argos.co.uk/static/Product/p ... FROTTO.htm

It's supposed to have a twist lock that allows panning but is absolutely crap.

I do have a little hot shoe spirit level - like the 2 axis one you've linked to. It was a free gift on a magazine! I quite like the look of the other one though.

Few things - my tripod has a level near the legs, which is pointless. I also have one on the head itself - more useful. The two never tell the same story.
Mostly with the DSLR, I've been shooting handheld using the AF spots to judge what's level. The medium format film body I've been shooting with doesn't have a hot shoe, so then I'll use a combination of the head level and my own sense of what's level.
If you're shooting on a tripod or a bean bag support, then they're pretty handy to get yourself spot on. Of course there's always shots that don't make sense with the camera dead level!

The other thing is are you shooting video or panoramas?
If it's for video work, then I'd suggest a video head. One that's fluid damped. This'll give a nice smooth pan and tilt, but the trade off is that you can't easily flip the head into a vertical position to do a portrait aspect. My head is actually a video head, and this is a draw back sometimes.
For panoramas, it'll help if you can rotate the camera around the focal plane - so if possible, make sure the pivot point of the tripod is under the focal plane of the camera. This becomes more of a function of the camera plate and the mount rather than simply finding a level.

I've used the level near the legs when setting up on uneven ground, and then the head level for shooting. However it has since sprung a leak and doesn't work.

My tripod head
It's very nice.

Mark