I often see Ness and Lucas get compared to one another. Though move-for-move, Lucas is better, for some reason I think Ness is the better of the two... it's like the way I see 'dorf and Falcon: Ganondorf has better moves, but Falcon just seems... better.
Anyway, onto Ness...
I think he's a great killer, personally, with the most reliable KOing throw in the game, powerful aireals, and a monster of a spike. Also, it's surprisingly easy to get caught in PKT2 if you don't know what you're doing.
The problem with Ness is that his attacks are either awesome or terrible. All of his smashes are bad (dsmash is an okay surprise move but the damage is poop), his utilt is good but I don't think his others are useful (dtilt seems like it could have some cool setups if it trips, but the range is terrible) and his B moves aren't very good either.
PK Flash is even worse than in Melee, PK fire is super laggy (but with big rewards if you land it) and though his PSI magnet can force some approaches it's not very useful. PK Thunder is useful and Ness' PKT2 is awesome but Lucas' is better, I think.
Great throws all-around, though, and all of his air moves are awesome. Fair beats shields and seems to chain into itself. Double-jumping while doing air moves is a great way to make them land. Speaking of which, I find bair super easy to land (compared to The Knee at least
) and it has a really nice kick (get it?).
Eeee... Ness is not so bad. He could climb to mid tier, maybe, but for now him and Lucas are at the top of low in my book.