There are two kinds of Adam Sandler movies: the kind where he plays a complete idiot and uses a (supposedly) funny voice through it all and the kind where he just plays a sweet doofus with his regular voice. Generally, those regular voice movies (Happy Gilmore, Big Daddy, and The Wedding Singer) are more broadly appealing than the funny voice movies, like The Waterboy. Although LITTLE NICKY is in the funny-voice category, it benefits from higher production values and a strong supporting cast and ranks as one of the better Sandler movies. That voice gets extremely annoying very quickly, though -- like after the first thirty seconds.
The plot is just an excuse, of course, for a lot of low-wattage jokes, usually repeated for no additional benefit. But the movie is enlivened by some funny cameos (Rodney Dangerfield as Satan's father, Reese Witherspoon as an angel, Henry Winkler as himself, and a surprise visit by a rock star). Sandler fans will enjoy it, but it will not have the cross-over appeal of The Wedding Singer.