My DD is 14 and has had homework since she was 5. The problem with starting them young (in my experience) is that by the time they are older they really see homework as a chore and become unwilling to do it just at the point where it becomes important for them to do it. In a nutshell: after 8 years of pointless homework they are completely turned off and start avoiding it (unless they're the class swot, or have a pushy parent standing behind them. (Sorry, pushy parents!))
My DD is just choosing her GCSE options and after a couple of years of arguing about/avoiding homework, she is beginning to see the point of independent, out-of-school learning and knuckling down again.
I support the idea of homework for secondary school children, who are old enough to properly research things, think independently, etc., but never saw the point of primary school homework, which required too much parental input and seldom seemed to be designed to consolidate what they'd learnt in class.
I agree with many of the other posters, when children are 4 - 11 they should be going home (or to an after-school club), having a snack, running around like mad things and relaxing.
The other thing that used to bug me was that I was a full-time employed single parent so my DD went to childcare clubs after school. I'd pick her up at 6 (hyper AND knackered simultaneously), take her home and then I was meant to sit and help her with homework when I should have been cooking dinner, bathing her and generally having some fun-time with her before getting her into bed at a sensible time. Homework used to put a ridiculous amount of strain on an already difficult routine.
I never got homework at primary school (in the olden days) and I can remember actually looking forward to starting secondary school, getting homework and feeling grown up. Now, by the time they're 11, most children are utterly bored with it.