Ferrell actually delivers some decent one-liners in his movies. 90% of Sandler's humor, even in his good movies, is from making faces and yelling in his angry voice. I think a previous poster was right, I don't think his schtick really got worse, he just never grew into something new and a lot of core audience kind of outgrew him. I think the edge in quality a lot of people would give something like Happy Gilmore is just sentimentality.
That being said, Ferrell is just as capable of delivering a turd of a movie (Land of the Lost, Semi-Pro).