To be honest, you probably will be.
There are 3 types:
The ones with glass in (made from the ends of milk bottles) that will give you thoroughly unpleasant results unless you're stopping right down.
The ones without glass that act like small extension tubes, meaning you'll probably lose the ability to focus to infinity.
The official Canon one that suffers from neither of the above problems, but you'll probably need to re-mortgage your house to afford one, if you can find one for sale!
There's a reason why most people who use old manual lenses go down the M42 route, it's a lot easier than FD. Personally, I'd go for option number 2, that way the quality can't be affected by cheap nasty glass (and they'll all perform pretty much the same, regardless of price).
Chris