In one of the DVD commentaries it mentioned that 2/3 of the SpongeBob fans are adults. Thought that was pretty cool
But... I doubt that continues with the "new" stuff. Starting with the SB movie & season 4, I can't believe how much it has changed. The stories are not as creative, seem more "childish" and IMO have lost the adult appeal. SpongeBob also has a different attitude and voice style now. He sounds and acts plain "silly" where he used to be more "serious yet sarcastic" sounding and more entertaining.
I am still a big fan of seasons 1,2,3 and watch them all the time. I am hoping the producers will get the message and bring back the old SB for season 5, but I am not holding my breath. At least no matter how many times I see them, the old stuff never seems to get old