Another fan for the following reasons....
We live in a hilly forest so just upgraded to a Beinn 20 small frame for DS1 5yrs, 110cm tall. It is mint. He whizzes up and down the gears, eats the hills up and the frame is SO light. He himself weighs only 19kg and standard 20" frame bikes we were looking at weigh in at aroung 13kg - which way to heavy for a wee boy to lug around the mountains. The new Beinn 20 weighs 8kg or so ...much better.
Trashing, well we have tried to instill in hil not to crash it down on the side where the gears are. However, like good quality adult bikes if anything is likely to break it will be a part readily replaceable. (DH is a bit of a bike freak and he would tell you himself...had he not been rendered catatonic from excessive excitement of back to back Tour de France and Olympic Cycling glory me).
Also, agree that they hold value well. This make regularly goes for £200-£220 plus postage on e-bay cao I watched them for ages! The new one was £250 with guarantee so not much in it really. As he is only just fitting it (it was a big step to go 14" to 20"!) we expect to get easily three years out of it and either hand it to DS2 or sell it on and replace.
If your DS has not had a go have a look and see if there are any roadshows islabikes are doing nearby and try one out for size and fit. The bikes really are designed for kids proportions and I think they are fab.
We do consciously ensure it is housed out of the rain which we never did with his first £50 bike and his second £80 bike - but then apart from leaving them ouside to get rained on(to be fair he is still only 5 and a bit ditzy that way) he has not wrecked either of the others. We have spent £130 on two bikes he has used to to death and simply grown out of.
If you boy rides alot or loves riding then I would say it would be the best pressie you could give him and it will sell on at a good value after wards.
Have we all convinced you yet??!! 