I'm just back from Vallnord on a freebie with Commencal. It's a beautiful country, the towns and mountains are stunning. It's definitely worth it for two nights. The trails are very bumpy though and after 2 days I was pretty badly beaten up, even though I was riding a very nicely set-up demo DH bike. I wouldn't want to ride for longer and was very glad to get back to my own trails and away from the braking bumps! I will definitely return though (it's only 4hrs from me) but will go for a couple of days maximum.
There are loads of trails that aren't marked too which were tough but nice and smooth! We were riding with the girl (SoSo) from the CG bike school and she showed us about. It might be worth getting in touch and seeing if she'd do you a days guiding. She can speak a bit of English and is fluent in Spanish, Catalan and French.
I didn't ride much that I'd describe as flowy and XC though.
Give me a shout if I can be of any help (hotel recommendations or anything like that).