I wouldn't say the Chiefs got a steal, per se. The 49ers and Chiefs just got a perfectly equal trade, in my opinion.
A first-round QB is a franchise QB that you build your entire team around, a la Drew Brees, Peyton Manning, or Aaron Rodgers. They're guys that, if they go down, the entire team probably tanks.
Alex Smith is not the kind of guy who puts the team on his shoulders and win games, regardless of his surrounding cast. I think he's a great QB who can win with the best of them, but I honestly don't believe he could do the things that the above guys can do. Peyton Manning and the Colts were perennial playoff contenders. When he went down, the Colts had a losing season.
But Alex is definitely worth that second-round pick. He's a great QB, and a big reason why the Chiefs are where they're at.