I'm currently the proud owner of a 4 year old dell precision M6600 and a 5 year old macbook air 11. Both used very intensively and both are in just about perfect condition, though the macbook does have a small dent where I knocked it off a table onto the floor (this would probably have killed a plastic cased HDD laptop).
I agree with AD though - the construction on cheaper products is crap. there was a time when you knew your laptop would be outdated in a year so there was no need to bother buying something well built, but nowadays it's worth the money to get something that will last
have just bought an HP envy 13 which looks superficially as well built as a macbook but only cost £600. if I get 5 years out of that, it'll be a bargain.