hi i bought the smc pentaxM 135 a few weeks ago as it was going cheap on ebay (still is I think), got it for €25 and i would not recommend it, especially if you mainly shoot video:
its only pros are that it's compact, light and cheap, but its a bitch to focus with dslr, really not as sharp as i expected, and has big lack of contrast. Image quality is good though, but i used it pretty much only for landscape. So if u didn't have anything else, id go for it cos its really cheap, but its going to be a pain to film and the result won't be too satisfying either, and i don't see why you'd use it with the lenses you've already got.
Sonunless you really fell the need to buy one now, like TacoLiger said, save up for a UWA, it's definitely worth it.
I'd like to see things from your point of view, However I can't seem to get my head that far up my ass.
BiTcH A$$ FooL