As long as you're sitting directly behind the net or along the sidelines from the 300s down, you will have a pleasant viewing experience at Centre Bell. The corners and all seats in the 400s should be avoided in my opinion.
I've only had seats in 321 once, for a St. Louis Blues game, but have been to more Canadiens games than I can remember including playoff games as well.

The only unpleasant experience I've had was sitting in section 406 where part of the ceiling hangs down and can keep you from seeing the other side of the arena. The 400s are also very cramped, you'll be too close to your neighbor it makes your trips to the concessions that much longer so you risk missing part of the game. You get what you pay for and that includes in the NHL's largest arena.