U-smash is really really good in Brawl.
It has great range (especially behind Mario), comes out REALLY fast, seems to have invul frames, it's great out of shield, and it reliably kills most characters at about 120%. The only drawback is that you must choose between using it for dealing damage, or keeping it undiminished for a kill. Keeping it undiminished fortunately isn't hard, and can be compensated for. Also, most people don't do this, but it is possible to hold the charge stance on a Dash Attack Cancel U-smash, and slide a pretty good distance. Anyhow, when I play against ROB for example, I always save my U-smash for the kill at about 128% or so. Against Ike however, I use to to punish him out of shield when he approaches. It's sick.
D-smash is also really good.
Comes out same speed as MK's D-smash, sends people at a fairly low angle. Very good for punishing from shield or spotdodge. Jab faking can combo into it sometimes. It's amazing for getting people away from you and off stage. While you can kill with this attack, it's so good for punishing people that you should use this for dealing damage. Btw, little known fact, but all of Mario's Smashes have more range than Luigi's. =)