I liked the earlier seasons also. But it was unsustainable like that. The boy, constantly chasing the girl, but never getting her... gets old fast. I also think they ran out of ´science´ material. There are only so many science jokes you can make to where they sound intelligent and funny, but are not so technical that they go over most viewers heads. The first few seasons hit on stuff like Shroedingers cat and the doppler effect and some stuff with time travel.. but there just isn´t that much past that.. more jokes about a paraplegic?
I do think the show has gotten worse, not because of the new cast members, but because of the rapid development of the 2 main characters who no longer feel like ackward geeks, and it almost feels out of place when they do have geek-ackward moments.