I don't think they are very safe to be honest; the few times I used them for what they are supposed to be used for, I was nearly run over. I'd stopped in the left hand lane and when lights were red I moved over to the right within the ASL becasue I wanted to turn right. WHilst doing so, the lights turned green and an Audi nearly ploughed into me. Cue the beeping and cursing and waving of hands and shouting "WTF are you doing, why are you in the middle of the road?"
If drivers knew what they are for, and respected that, and still drove with due dilligence then they'd be a great thing, but we all know that's a rarity.
As for MC, I don't see the harm if they've filtered to the front and take care whilst entering them, I used to sometimes, but I would always do a shoulder check and "give way" to any genuine (cycle) users before I pulled away or turned. If there was more than one cyclist in the ASL then I wouldn't normally go in there