I started watching it from the beginning a couple weeks ago. I'm on Season 3. Last episode I saw was Sweet Dee Dating a ******** Person.
I'm not loving this third season as much as the first two, but it's still really funny and enjoyable. I loved season 1 when it was just the four of them. When Danny DeVito shows up as Frank, I was skeptical at first because I didn't like how their dynamic changed, but I don't mind it now. Danny DeVito is great and I actually think he brings something extra to the mix.
However, I HATED, HATED, HATED, and STILL HATE that they revealed Frank not to be Dennis and Dee's father. I wish they hadn't done that. I liked Frank being their father, lol.