Hi, yes i just started using M42 lenses on my 550D and i like it.
They're cheap but (depending on what modell and brand) still very good.
The minus points are the missing autofocus and no optical stabilization -> requires faster shutter speeds (~above 1/60) or tripod. If you're looking on Ebay for a vivitar, zeiss or cosinon (all good brands) you should demand good photos and watch out for scratches or fungus in the optical elements.
Have fun
PS: older people always have some good old lenses in their attic. just look out for screw mount and carry an adapter for testing out