I've had a quick go with FD lenses on my 5D and have to say that my preference remains in favour of M42 & Ricoh mounts. I've kept an FD f1.8 50mm for now with both types of adapter, optically assisted and mechanical, but I found the actual mounting process quite clunky and it's never going to be the lens I reach for first.
That said, I know that people get good results with FD lenses and that there are both excellent and good value examples out there, as well as companies who can undertake a proper conversion to EOS mount.
In terms of speed, I'd say that the majority of MF lenses on kneebay are not rocket fast but f1.4 lenses in 50/55mm are neither rare nor expensive. I have an Kneebay search for f1.2 lenses (not f2.0) and regularly see examples in this speed at prices much lower than would be expected of an EOS mount.