I do commute in using the part of the A23 which experimentally allows motorcycles etc to use the bus lane. I don't see that many cyclists of motor cyclists, since I generally travel late to avoid the busier traffic, however the motor cyclists I have seen don't take an awful lot of care of me, and one damned near did hit me today, not that I think he realised this, as he was busy jumping some red lights ten seconds later.
At peak times I've noticed that as many motorcycles as cycles will often fill the cycle refuge between the lines, where there are advanced stop lines. Often when I go up into that area, and there is also a motorcyclist there, when the lights change, frequently they will turn left across me, or suddenly realise that I'm not actually going that slowly, and they can't easily do this (often they will not have been indicating).
This is more a case of bad road use, which clearly is not the exclusive province of motorcyclists, most other road users are guilty to various degrees, but it's not particularly pleasant to be waiting at some lights with a motorcyclists sitting next to you, gunning his engine, and you know that he's going to try and turn left across your path when the lights go green. Of course motorcycles shouldn't be there anyway, but the ones I've seen legally using bus lanes don't seem to be a good demonstration of careful usage, since most of them seem to see it as an excuse to speed along a clear route.