I've got the MSI Megasky one, and its absolutely junk.
It came with a little aerial so that you could use it for laptops, however unless you live approx 10-20ft away from the antenna its signal quality is so poor you wont get anything.
I plugged the aerial on top of the house onto it and the quality of the picture at first seemed great, however sometimes the aerial signal seems to go a bit crap and you end up with a second or not of picture freeze, this happens more than i would have hoped. The freeview box that we have and uses the same aerial detects a high quality signal and never ever jumps.
Also ive had problems with the megasky locking up the computer sometimes as little as 2-3 mins into viewing and sometimes after an hour. Again it seems to be when it thinks the signal is degrading (although again the freeview box never has these problems). For those interested i have tried this on 3 computers.
My A64 3700 - 2GB ram - windows xp
My Shuttle Athlon2100+ - 1GB Ram - windows media
My 2.5ghz P4 Celeron - 1GB ram - windows xp.
You would be better spending your money on a PCI version.