I've been mtb'ing for about 10 months now and I have been in a number of places around Portugal and the thing the really pops to the eye is the explosion of mtbikers here!! I mean, 2 years ago there were some (very few) people doing it but this last year or so lots and lots of people have been joining our ranks! We even have an all-Portuguese by-monthly mag exclusively dedicated to mtb!
Riding here is excellent because there are so many wild places to do it... He have got mountains, most of which are natural parks (and mtb is considered an ecological activity, bikes are treated as pedestrians when it comes to restricted trails, roads, tolls, etc.); we have a (very big) beautiful coast line; we have the planes of Alentejo (big spaces, almost all plain and mostly uninhabited) and many other places...
The mostly warm climate makes for little need of heavy winter clothing, but when the rainy season gets here, look out!!! Rain here is not like... say... England, here when it rains it usually rains heavy and for days!! (Not being a particular fan of extremely muddy rides I could do without it, but that's what we have... :=))
All the bikers are really friendly and help each other out whenever needed. Even roadies are friendly to us! ;=)
All in all, there are some absolutely incredible places to go to and I've been to some with great rides and breath-taking scenario...
The competition scene is getting into its legs, with a national championship up and running, but I'm really badly informed about it (I do it for the fun and don't care about racing... except racing down every hill I can find... :=))
Then Nuno added:
Anyway, my strongest advice is: If you come to Portugal to MTBike you *MUST* go to Gerez! It's a natural park and one of the most beautiful and quiet places I've ever known... and lots of down-hill courses also :-)