As much as Elmo is the focus, it seems that Grover is one of the evergreen (or, in his case, everblue) standbyes that make SS what it is. I really think that Eric Jacobson's performance as Grover helped the character to relaunch his popularity.
Grover seems to be a huge favorite with older genereations, but younger kids like him too. What I think is lacking is more skits with Elmo AND Grover. Like the one where he was the singing telegram monster's assistant. I always liked how Grover seemed to be the big brother of the 2, and trying to show Elmo the right way to do something, while Elmo does his own thing, and it works out better. Hey, Elmo's on most of the show anyway, why not pair up with another character?