I've read with great interest the various threads on M, A, and other older lenses here. Part of why I chose Pentax as my dSLR is because of the backward compatibility with older lenses.
When I got my K-x my sister and B-i-L gave me three lenses that his parents had given him along with a Pentax K1000,
I took the Pentax M-135/f3.5 out for spin today at my daughters Softball game. Wow, that's fun!
Sure it was slower to focus but the built in hood, the solid feel, the smaller size, and the control it gave me in setting the aperture, etc. was great.
One advantage I noted that I have not seen referenced here in my reading is that when I manually focus I can get sharp pictures THROUGH a chain link fence. Given that nearly every BB and SB diamond has high fencing around the bating area, at least, I found this a nice plus in using the Pentax M135 today.