Basically every OM lens betters every other possible M42 lens that I have ever seen.... so no sense adapting inferior lenses to the OM system when the OM lenses are simply better...
maybe there is some special M42 lens out there that would make it worth considering, but it's easier to adapt to EOS as far as M42 goes.
Because the M42 lenses are far cheaper (for some)
And I have a very nice set of M42 lenses:
16mm Zykkor
18mm Asanama
200mm f3.5 Vivitar (a very fine optic)
400mm f6.3 (probably not a very good lens, but better than nothing)
Lots of OM "T-mount" (M42) adapters on ebay for dirt nothing. Just make sure it is OM mount and not 4/3 mount. For some reason the eBay crowd keeps callinh the 4/3 mount OM 4/3.
I have 2 of them. One is actually a reversing ring that also has 42mm threads on the ID.
I did a search on ebay and found several for about $12-$15 bucks. I found this one mounted on a Macrotel 150mm f/4.5 Bellows Lens for a steal. If you ever plan on getting a m42 bellow unit this would come in handy.