xs4all is good. But I'm very very happy with demon(also a quality provider)
You might also wanna check out
www.tweakdsl.nl Also many good stories about them. Bad thing is, they sell only 2 year contracts...While demon and xs4all offer one year contracts.
On the 3 years that I have demon, I have NEVER been down(only normal maintenance, and you get emails a week upfront for this downtime)
Andre