All in all, each of the nine classes have their unique strengths and weaknesses. Some are best in defense, some excel in offense, some can do both depending on how they are played, and all nine feel useful without any obviously overpowered classes. Class balance is the most important factor in a game like this, and I must say that Valve has got it so close to being just right that it's clear they've really played their own game over and over again, polishing things until every class feels useful, yet fair. A rare achievement.