It's just good. The plots are amusing, just the right quirky blend between cheesy and clever. They tend to mix dastardly supervillain plots with something not supposed to be evil (but totally is, once you think about it: TV psychiatry, the frat system, boy bands, tubas).
There are also a lot of references, quotes, and sometimes self-conscious theft (Holy Terra Cotta T2, Batman!). It's hard to do that and not be either as pretentious as a TS Eliot poem or a worthless knockoff, but they pull it off. The references are fun if you get them, but the show isn't the sum of its allusions.
The dialogue is criticized for being too crowded or fast, but seriously? It's fine. If you don't catch it the first time, watch it a second time. Or a seventeenth. Or, in my case, a thirty-ninth. It's all good. :D :D :D :D
Actors: Matt Keeslar is so the character that I actually wasn't sure that the actor on the other side of the quantum singularity in the last episode was the same until my fourth time watching it. Natalie Morales's facial expressions are fantastic (and now make up the bulk of my usericons). And they're both eye candy. Liek woah. And even better, the show manages not to be pervy about it--except in episode 9, where it's deliberate.
What makes it work best, though, are the characters. MM is as serious as Mao Zedong's heart attack. WW is snarky and less obviously straight edge. Since Wendy is the main character, she develops a lot over the series. MM is static, but he's supposed to be. Their partnership, though, is not.
The situation is completely unrealistic (um, that's the point?), but somehow that doesn't matter when Wendy's giving her "Boss, if you're hearing this..." speech. It's also nice that the writers don't imbalance them. MM's the boss and WW's the trainee right until the end, but he doesn't do everything and her development doesn't edge out his accomplishments.
Minor characters: IDA. OMG. HEART. Lacey could easily be annoying, but she isn't--and another thing is, I like the fact that Wendy and Lacey are actually friends and not frenemies. Tyler is as awesome as he's supposed to be, although it was weird because I didn't believe that he wasn't Wendy's brother until after they'd had sex three times, and even then I had trouble...
Quibbles: special effects suck a couple of times. Oh, and I get that they probably did this for effect and because Natalie Morales is shorter than all the others--and of all the unrealistic things to pick on, it's the least outrageous--but...three inch heels? Seriously? ALSO, THE FACT THE SHOW WAS FRAKKING CANCELED.