Before I acquired this lens, I had never seen one in person. The Super Takumar f2 (62mm or 67mm filter thread) is much more common than this 49mm filter size version. Never used the other version except the f3.5 SMC. The faster aperture is very helpful in focusing, since the view finder is much brighter. Initial impression seems to be a well rounded lens that is satisfactory in performance. Like most lenses, contrast is low wide open at f2 but not unusable. Further use and evaluation is needed.
This lens was bought at about the same time as the Leica-R Summicron 35mm f2, so I was very interested to see how they compare. I ended up using the Leica more, that should tell you something :-)
When time permits, I will do a more thorough comparison.
[Update March 15, 2009]
So I have used this lens a few more times, and I must say that this little lens can really grow on you. The SMC really is fantastic and flare resistance is amazingly good; better than the Leica version. When stopped down to about f8, the lens is sharp across the frame on a full frame 1Ds.