OP when you say "Is Silva actually that good", how good do you mean? Do you mean world class (Messi Iniesta) or the class below (Robben Sneider)?
I've watched a lot of Spanish, Champions Lg and Europa Lg footy and Valencia have been on a fair amount so it's fair to say i have seen a good amount of Silva (from what can be seen on the tele).
He's sometimes very good - however, not at Villa's level - can score, can assist, keeps the ball very well, likes a shot from outside the box, most things you'd expect from a good froward. But sometimes you don't really notice him make an impact on the game. Which is where he isn't world class.
Time will tell. The fast paced Engish game may or may not suit him, but he is better than what we have (Bellamy/AJohnson!)!