I noticed that nobody posted a Bowser topic, mainly because so few other people use him, so it's understandable.
Bowser is my main fighter, I probably use him about 85% of the time I play Brawl. I'll list as much as I can about him.
Bowser has recieved a negative judgement by many gamers for his lacking skill in Melee. In brawl however he has undergone many changes that have made him a more competitive fighter in the game. For example, he is much faster than he was in Melee, (just FYI Bowser is faster than Snake, Ganondorf and Dedede, he is not the slowest character in the game. He is still however somewhat slow compared to Donkey Kong and a lot of other characters.)
One thing someone will notice about playing with Bowser is that he is very powerful. His A smashes prove to do a lot of damage and can be very destructive if performed correctly.
However many gamers who try Bowser make the mistake of only using his smash moves for damage and get pushed around a lot. It is important not to get obsessed with his smash moves. I find that his most reliable move is his fire breath (B standard). Bowser's fire breath can be a real pain because it catches everyone in it and unless you are a character like Kirby or Jigglypuff you will have a hard time escaping it. I have worked up to 75-80% damage on many people with the fire breath alone. If done correctly, a fire breath can also be an effective form of edge-guarding.
Bowser's next best move is his spinning shell move (B up). This move is suprisingly quick and a great move for escaping a jam as well as a way to keep opponents in the air.
His other great move is his new body slam move (B side). This move is a great move because it does a lot of damage and opponents cannot to much to fight back damage from this attack. The best time to use this attack is when a character is either guarding or trying to repel your fire. The time it takes for your opponent to finish the move is usually quick enough for you to grab them and perform the slam. Bowserciding is a technique where you take your opponent off the edge with a body slam. It can work very well for evening out lives in stock as well as wining you a match in some situations.
Bowser's smash attacks should be used at the right times, such as when a character rolls into you or when another character is being trapped in another character's attack. Be careful when using these attacks, they make him vulnerable because of slow recovery. Same goes for his sitting slam (B down)
On the whole. Bowser is a difficult character to learn how to use, but he is not nearly as bad as many people make him out to be.