There have been several threads about the halo on here that might help you:
http://www.quiltingboard.com/search.jsp?q=halo&u=
I wouldn't buy another one - I have the round one and the square types and they're just a pain in the neck to use.
The bed of your sewing machine must be perfectly flush with the cabinet or the halo will catch on the corners. You have to use repeated pinching motions to pick up the halo and reposition it - this is hard on your hands. There is very little space to work in, so if you like to do big sweeping swirls or feathers, your motion is interrupted to move the darn thing out of your way.
I think I've read from a few people that like them, but the consensus seems to be that it's more trouble than help.