From a personal point of view I think they're extraordinary value for money. Not only is there the entertainment value of building the model and its eventual running, but the cost/quality looks to be good too.
The best British model railway manufacturers manage this in spades and the worst can make a tenner look exorbitant.
In terms of a cost comparison, it makes HO seem pricey to me - I've spent that sort of money on German-made kits (before I had a mortgage though!) and, yes, I think them good value for money too. (More on my website as it's a bit O/T for here. Just in case you're interested!)
Steph