Personally, I prefer items off.
It actually depends which items...
I'm okay with most of them, but the Master Ball, the "rock sword that makes tornadoes" and some that I might forget are kinda annoying/give free points for doing nothing other than grabing and use it. Tough, the game is boring without items.
I prefer items to be on, but my brothers always turn off items they don't like when we play.
Such items include pitfall seeds, guns, weapon parts, warp stars, etc.
Basically the FUN items. Thank god they keep Smash Balls on.
Depends who I play with, and how we want to play. Though many of my friends prefer items off for skill purposes, and I agree with that at times, but when I wanna have fun items are a must!