Originally posted by MXLX Well...If I'm honest...
I happened upon a 135/2.5 for a song as well.
The K 135/2.5 is superb; I use it for indoor concert photography at a small venue, and it yields great closeups of musicians. Sharp.
The K 200/2.5 . . . yes. I used to use it at indoor middle school band concerts, hand-held, and it performed really well.
The K 120? Sigh. I don't have one. But I do have the M 120/2.8 and I like it a lot. Not sure what a comparison might reveal, but I can't think of anything faults with the M.
I think overall I have more K lenses than I do M lenses, but I don't think I'd balk at an M. I had a really sharp M 28/3.5, though, that I sold after shooting a couple years with the K 28/3.5. I can't remember for sure why I kept the K and sold the M, but I think it may have had something to do with the bokeh. There was something about the K that struck me as just ever-so-slightly more to my taste than the M. (Of course, if I were to make the comparison now, or three years from now, it's possible I may have opted to keep the M. Who knows.)
Anyway, you're going to love the K 135/2.5, and I'm sure you'll love the K 120, too.