I just watched the 2nd episode and I can see why people love the show. Rob Lowe's character seems like a toned down version of his Parks and Recreation character and that's a good thing. I also like Josh and Leo, though C.J. and Toby have yet to impress me that much. As for Bartlett, I'd vote for him. The only one I can't stand is Mandy, time will tell if (a) she'll grow on me, (b) she's not supposed to be likable, or (c) my opinion is based on the fact that I like Josh and therefore don't like his ex.
Only about five people in the whole world like Mandy, so don't worry about it.
She ran PR for a President who didn't like being managed by PR people, so she doesn't have much of a role on the show beyond the opening episodes. She typically just shows up for one or two scenes per episode to justify being part of the main cast, and is then unceremoniously dumped.
Toby is an excellent character if you like the cynical, brooding sort, and he works as an excellent foil for Bartlet. They love each other, but they have a way of getting underneath each other's skin and that leads to some explosive situations.
Just started final season. I'm probably in a minority but I've enjoyed the latter non-Sorkin seasons more. 4,5 and 6 have been particularly enjoyable.
I have a bit of a soft spot for season 5 because it was the first full season I watched, but it's easily my least favourite of the seven. Pretty much everyone is out of character, it's unusually episodic, and the stories are just dull. Thankfully, the election storyline in the final two seasons gave the show some much needed direction and I think the show ended on a high.