I've used them before and they're pretty good. You will pay more than face value of course! Early games in August are showing at 50 Euros, which isn't bad.
I've just moved to Amsterdam and am thinking about becoming an Ajax member. It will be useful for when Brighton make the Champions League
Apparently, it takes 6 weeks to process an Application for memberships (and we moan about the Albion!).
Depending on what game you want to go to, you may be able to get tickets on the day. I have done this a few times. If you're looking for tickets for teh big games, PSV and Feyenoord, you probably won't have much luck.
If you let me know what games you are interested in, I can have a look into sorting tickets out for you this end to avoid you getting clobbered by a ticket agent.