The problem is that in Spain, Nikkor lenses are ridicously overpriced (I paid 170€ for the 50mm, while in the US it costs only 100$; 170€ are 205$...
). Also, while the Nikkor 70-200 HSM is widely available here, the 70-200 non-HSM, wich costs a lot less but offers the same optical quality is out of stock. So the only alternative left is the Sigma, wich costs a little less than the non-HSM, and A LOT less than the HSM (I can buy the Sigma for 750€; a USED Nikkor HSM costs 1200€...).
About optical quality, opinions vary a lot. Some people say it's great, some say its junk, some say it's on par with the Nikkor, some say it can't even be compared with it... It seems that the only way to find out the truth is going to the shop and buying it.