I have RMA'd several motherboards and video cards to Asus. They were fairly reasonable most of the time. The one time I had difficulty I was able to get into an email conversation with 1 particular person and it was squared away fairly quickly. Turned out the board I was trying to RMA was temporarily out of stock. When I did email them, they almost always got back to me within 24 hours (sometimes less than an hour!).
The items usually took about 3-4 weeks to turn around. The one that was a problem was just over 6 weeks. And that's going from Canada down to the US and back.
All in all, I'd say my experiences with Asus RMA were ok. Not spectacular, but not horrible.
That said - I've never done an RMA with any of their notebooks, so that could be a completely different animal.
EDIT - I should mention that if you bought the notebook from a good retailer that has a strong relationship with Asus then you're probably better off going through them. If you bought it from someone who's turned out to be unhelpful and a poor retailer, then your time is probably better spent doing it yourself.
Even if you built the notebook yourself from scratch, if you can prove its a problem with the shell (and not RAM, CPU, HD etc) then you should be able to easily RMA it through either the retailer or Asus. That said, if you did build it yourself you will likely have to answer all the annoying questions/suggestions they'll have about your troubleshooting process. That's what I had to do with all the motherboards I've ever RMA'd - it was usually at least 2 or 3 emails traded before they'd OK the RMA.