I use several M42-to-EOS fotodiox adapters, bought from the fotodiox store in ebay
they work perfectly fine, many people say fotodiox is one of the best brands for lens mount adapters
I've also used a much cheaper chinese version from ebay ($10 or so), and, while it was ok and I couldn't find any differences in picture quality (infinite focus ok, no aditional wiggle, etc), I got rid of it and now I stick to the fotodiox adapters: they're not that expensive anyway
em... it's just a mechanical adapter, no electronics whatsoever
all my lenses are vintage and manual, so I don't need AF for anything, but I hadn't even conceived there could be M42 lenses with electronic control: really, on this screw mount? I didn't expect it
some adapter has electronic chip inside. You can use the body AF focus assist, same as with original lens in manual mode. If you set the orig lens to manual mode and use the focus ring the body can beep and assist AF module if something is sharp. This feature you can also use with the adapter according to the manual of the adapter.