I loved it, and really wish they'd done the fourth season. There were heaps of nods to previous TF series, generally strong characters (Animated Ratchet in particular is pretty much my favourite Autobot character to date) with nice 'twists' (i.e. the Constructicons wolf-whistling at passing convertibles) and usually string writing/storylines, the toys were a mixed bag but with far more strong figures than poor, and it was just well thought out in general. You could tell it was a series done by fans, for fans, while still being accessible to newcomers to the franchise.
Like just about everyone I was put off at first by the art style, albeit intrigued by how they could possibly manage to get the character designs into toys, but one day I was bored and started watching the opening three-parter and got hooked.

I'm quite enjoying Prime, the designs are interesting, the combat sequences are pretty good, and there are some good characters (Arcee is great, Starscream is pretty good though falls short of Animated Screamer IMO) but for me there's nothing quite like Animated. Reading what they had planned for S4 is almost painful.