I don't have kids and I probably never will, but here's my opinion.
We got our first family computer when I was about 7. I got my own first computer in my room when I was about 13. My own laptop for my 16th birthday, and from there, I obviously buy my own computers now.
I love my parents immensely and I thank them for spending their hard earned money on buying us that first family computer. We were broke, we always have been, but they spent money on that computer for me and my sisters, so we weren't "missing out" on anything, etc. Truly great people.
But I've learnt a lot from the way my parents did things. Mainly, that if I were to have any children, there'd be no computer/laptop in their room until they were 16, and time limits would be enforced to maybe 1 hour a day to begin with and then as they get older, 2-3 hours, no more than that.
The second I had my own computer in my own bedroom that was it. Homework went out of the window. My hobbies and interests went out of the window. I was actually a HUGE reader when I was a kid, I read the first Harry Potter book when I was 7, and got bought a new book bought from and with my mother every Saturday that I would read within 2-3 days. This stopped when I had a computer in my bedroom... everything stopped. It wasn't computer games, really, as I've never been a big gamer, just mindlessly sitting on the computer on MSN/forums/chat rooms/etc. all day every day. My social life, hobbies, interests etc. all disappeared. The computer was my hobby.
To this day, I still spend over 8 hours online a day. I'm pathetic.
I definitely wouldn't want this to happen to my kids if I were to ever have any. A shared family computer for 1/2/3 hours a day is good enough. Forget what advertisements say, take it from someone who spends way too much time on their computer - and it's still less than I used to - it's not healthy. Access to the computer and internet should be a supplement. Don't let it take over your kids' lives.