From an e-mailer in the biz:

Speaking as a comedy professional, I can tell you that she was quite good. Her material was B-plus at best, but she was remarkably at ease, considerably more comfortable onstage than many talk show hosts, especially at the beginning of their careers. Does anyone remember the quivering mess that was Conan’s debut? Sarah knew where the laughs would be, didn’t oversell, and didn’t panic when she didn’t score.
What I am wondering is, why in the world would she do this? She’s spent months honing an image as a serious political player (to whatever benefit, I am not sure) and then she takes the opportunity in front of a national audience to audition for the role of a talk show host. (While ironically, Mitt Romney was on Letterman auditioning for a much different job.) What is she trying to say? “Look, I’m still fun!” Has she spent a year as Serious Sarah, and just now realized she doesn’t love it, and wants to be Star Sarah?
The funniest line of the whole segment was when Jay said, “I’m going to introduce you the way I would any new comic.” Jay stopped having new comics on the Tonight Show a very long time ago, which is seen by many as a big reason stand-up comedy has been dying. Maybe when Sarah hosts her show, she can reestablish Johnny Carson’s tradition of giving up and coming comics a place to launch their careers.