First, the FD-EOS adapter will cost you an additional $40.
I am a fan of manual-focusing lenses.
Here are my 2cents:
1) the SSC coating is better - I do not have the FD 100mm f/2.8 SSC, but this lens may be ok because of the SSC
2) As in the previous post, with these MF lens, you can only work in Av mode
3) I have the FD 50mm f/1.4 and FD 300mm f/3.5, and these lenses are OK
4) People who use MF lenses only purchase the selective MF lenses known for their superior optical quality
5) To help you with manual focusing, you may need to spend another $40 for a split-screen focusing screen
So, using MF lenses may not save you money
Here are a few MF lenses that I own:
1) Canon FD 50mm f/1.4 SSC used with FD-EOS adapter
2) Canon FD 300mm f/3.5 SSC used with FD-EOS adapter
3) Nikon 50mm f/1.4 AI used with Nikon-EOS adapter
4) Nikon 80mm f/2 AI used with Nikon-EOS adapter
5) Helios 50mm f/2 - M42 screw mount - used with M42-EOS adapter
Canon 40D / 1DM3 / 1Dx / 400mm f/2.8 II / AlienBee B400 / AlienBee 22" Beauty Dish / EF 1.4x Extender / PocketWizard Plus III