This post actually inspired me to create an account so I could reply. Hi, all.
I've done a fair amount of macro work with a few different Canon manual focus lenses including the following: FL 55mm f/1.2, FL 35mm f/2.8, FL 50mm f/1.8, FD 50mm f/1.8 and with an Canon FD/FL 25 extension tube in the optical train. My experience is such that I have found that these lenses are amazingly sharp on my 50D but have a serious depth of field issue. The close focus average is within 1" of subject. At that distance, dof just doesn't exist and it, at times, can leave a large portion of the image well out of usable focus, unless the subject matter is flat in nature and composition.
The EOS/FD adapters vary greatly from one to the next in terms of optical quality. I use an STPPO (?) mainly, but have also experimented with a Kenko which has a larger opening but lacks the fit for any thing other than the FL55 f/1.2. In other words, that adapter doesn't work on any of the other lenses I listed. Which is unfortunate because of the size of the Kenko opening allowing for more light throughput. Between both adapters, I see no real benefit for sharpness in one or the other. But the STPPO does seem to exhibit more color fringing with blue highlights on most bright sources of reflection compared to the Kenko. This could be a result of many things but all in all both are very usable.
My recommendation would be to save more money and get the EF 60mm Macro to start. It's a little more pricey than any of the manual focus options but the AF, clarity, and DOF are much more appealing overall. It can be used as a standard prime 60 - an added bonus which none of the manual lenses offer consistently as the ability to infinity focus isn't always going to work.
This was from yesterday: Romanesco - with Canon 50D, FL 35mm f/2.8, Canon Extension Tube FD 25, STPPO EOS/FD Adapter