Big Al is a 20-10 big man with good size for his position. He can play both PF and C reasonably well. That makes him a really solid building block for any good team.
The bottom line is, unless a guy is a total defensive liability (re: Zach Randolph), if he's giving you 20+ pts and 10+ rebounds a game, he's a really good player.
Now, if you could combine Dwight Howard and Al Jefferson - Superman Defense with Awesome Post Offense...that would be magnificent.