I've been watching Grep on a regular basis since day one. I still find them funny for the most part.
For me, the Grumps are at their best when they're reacting to the games they're playing. However, they're starting to spend more time going off on unrelated tangents, practically ignoring what goes on in the game. I often find myself scratching my head thinking how in the world they could have possibly missed this or that crazy event that literally just occurred before their eyes.
This may be due to the possibility that the "newness" factor has worn off for both the Grumps and their viewers. I think Jon and Arin were still in "entertainer mode" during the early episodes when things were still new and uncertain, but now it's just part of their daily routine. Consequently, they've begun acting more like their normal selves outside of Youtube Land. Also, the viewers have become familiar with the general routine of the show as well, so it's become more difficult to elicit the same hilarity as before. Jokes tend to be funniest the first time around, after all.
Also, is it just me, or are they bringing up gross and unappetizing subjects more often? I like to eat dinner while watching episodes, but it can be difficult sometimes when they're talking about poop, jizz and "butt babies."