Its essentially just a piece of metal that sets the lens at the appropriate distance from the sensor, as long as it says it will focus to infinity (and does when you get it), you're good. Novoflex makes adapters for K mount that have an aperture lever (still manual, it approximates stopping down the lens, but doesn't have actual defined aperture marks), which runs close to $200.00. If you need an m42 mount, just get the m4/3 to m42 adapter - anything from ebay will do. (there is no point picking a particular brand, they are all machined metal extensions, nothing more, nobody has ever complained about a particular adapter from ebay that I have read here or on dpreview).
Actually that can be a big problem. Focus beyond infinity may sound good, but in practice it makes for blurry photos when you try to catch something far away in focus and the infinity stop on the lens doesn't correspond to actual infinity focus. Ask me how I know... Given what I know now, I'd buy the Novoflex adapter in a second if it ensures perfect infinity focus, and if it fits perfectly. I've spent about that much in money, and much more than that in time making the cheap adapters work properly.
My Pentax adapter focuses beyond infinity. I do not rely on the focus stop; I line up the left hand 8 on the DOF indicator with infinity, as if it were the focus index line. In fact, that 8 holds good for any distance. You just need to know your tools.
What do I lose for focus beyond infinity? Two centimetres at the near end. I can live with that.