i'm not really sure how much you would use it..
but i know from having played with them before, that if it was in my budget i'd probably already own two.
I bought one a couple of months ago, after asking this very question. I haven’t used it as much as I thought I would, but when I have used it it’s been great fun (which is what it’s all about). It is a bit of a one trick pony but it’s a good trick.
Was really useful when I had to do that heavily sustained main riff from "The Cutter", but had little use on guitar since. Still, I can get some wicked harmonics on bass that has kept me from selling off the thing altogether.
I say buy one, they are really fun. But as someone said before the fernandes sustainers are cool as hell too. Plus with the sustainer your right hand can probably do alot more musical stuff cause you would be able to still pick the strings.