It's a hard decission, and of late, we've had to end up with William in our bed, as he wakes in the middle of the night and won't go back to sleep, so he comes in with us.
We have a king size bed and Will sleeps in between us and just has our doona on him, but he sweets and wriggles and ends up sideways, it's so annoying, but if it's the only way to get them back to sllep then I say do it, it can't harm them.

But from the word go, my kids have always just slept in their own rooms and encouraged to stay there, but when you've been up 3-4-5 times a night, and you are just so tired you bring them in with you, they stay asleep and you get a semi sleep too.