As long as you have a 6 pin firewire port on you pc it will be fine. Most PCs may only have a 4 pin one but I know mine has both. I am not sure if the iSight will work if you use a 6pin to 4 pin cable or converter (as it may need the power the 6 pin supplies).
On the PC, the video will work but it cannot (as far as I know and last checked but there may be a fix out there by now) use audio. You will need to use a separate mic.
Otherwise it is fine.
It does seem like it is an expensive option for a PC. They may be impressed with the iSight and iChat but remember you can't get iChat for the pc and the other clients bite if you ask me. It seems as though it would be a more expensive and complicated option for a solely PC user to get an iSight. They would be better getting something that works straight out of the box and is fully supported otherwise they might even start blaming Apple for "products that don't work" even though it was not made for the purpose (use on a PC).
Get them to buy a Mac mini then iChat and the iSight will be fine!!