I had the injection with ds (hospital birth, ventouse delivery) and delivered placenta within 10 mins. I also had it with ds2 (homebirth), and placenta took over 1 hour to arrive. Luckily, I had a very patient and capable midwife who could see I wasn't bleeding to death and so left it as late as possible, although she was picking up the phone to call an ambulance when the placenta finally decided to put in an appearance. I have to say that the labour pains for the placenta were far worse than for ds2 - up until then it had been an easy labour.
I reasoned that I'd had such difficulties as the syntometrine had contracted my womb too quickly and my placenta got stuck. So with dd (also homebirth) I decided against it. The placenta was a little tardy again - about 15 - 20 minutes, and unfortunately didn't come away cleanly (as it hadn't with ds2 either) and the midwife had to fiddle about a bit to get the straggly bits out (sorry if tmi!) Not a very pleasant experience but better than hospital and surgery anyway. I think that I obviously have a problem delivering placentas whole, as they seem to be a bit ragged (not sure why as neither ds2 or dd were overdue), and that the syntometrine makes no difference to this.
I've gone into the gory details of my experiences just to outline that it's never that straightforward, and I can't say that everything went swimmingly when I didn't have the jab. However I do believe that when I had it with ds2 I was in very real danger of a retained placenta, whereas without it I delivered the placenta much more quickly, and with much less pain.
I think (and I did research this before dd) that if you have as natural a labour as possible, with minimal intervention, then there is no need for the jab, and it could even cause problems. However, the more intervention you have, the more it is likely that your body won't respond how it's meant to and find it difficult to complete the third stage without help. I said on my birth plan for dd that I didn't want the jab, but obviously if there was a problem and the midwife thought I was losing too much blood, then the midwife shouldn't hesitate to give it.