I was refferring more to the switching options you get, rather than the actual quality of effects. A multi-effects unit that has silence between patches, or is difficult to operate with your feet, is fundamentally flawed, with respect to gigging, but maybe that's not it's intended purpose.
As far as the ME50B goes, I wouldn't really know! If it's in "stompbox mode" or whatever it's called, then you would expect that pressing the same button you pressed to turn on an effect, would also turn it off! If not, then, yeh, that seems crap, but, it doesn't mean all multi-effects are fundamentally flawed.
I find that having effects in a loop, whether they be multi effects or singles boxes, is useful, as you can take time setting up a sound before you kick it in.
It's only my opinion, but I think that multi-effects are only worth buying, if you need to switch between different complicated effects settings. For example, if I use 3 different gain settings on one of the dirty patches on my Zoom, it would take 3 copies of the same pedal, with different settings, in a midi controlled rack, or in a looper on a massive board to replace the zoom with single pedals.