Its great fun - the results you can get are stunning. I have a cokin version but they'll do essentially the same thing.
Is that the square version?
Yes it is - if anything the Hoya is better because there's a lot less/no chance of light leaks.
Im actually looking at the Kood R72 because its a bit cheaper, but I cant find many places that sell its accept for Surrey photography supplies and I have no idea whether they are decent or not :shrug:
I've got the Kood R72 here... My girlfriend bought it for me a couple of years ago but it's not seen much use since I've upgraded stuff. It'll currently only fit on my 85mm and 100mm so the use is a bit limited.
I've noticed a 'hot patch' in the middle of the images at these focal lengths but I never noticed it on 'kit' lenses.
They're not that expensive so it's got to be worth a punt.
Si
There are quite a few on the 'bay for £15 for 58mm ones - they can't really be that bad. Although they don't seem to be branded.
I use a 58mm Kood R72 on 120 Rollei IR film..
All I can say is it works perfectly and I haven't seen any patchiness, bit of vignetting maybe but that's it.