さすが (sasuga) as opposed to やっぱり (yappari)

I was just using 「さすが」 in an email and realized how often I hear this word used.  Usually when I hear it, it seems like the meaning is somewhere in the vicinity of "Ah, just as I'd expect from you."  I checked around a bit, with people I know and on the internet, and it seems generally to be a complimentary term.

A great question came up on a few websites.  What's the difference between さすが and やっぱり?
When I first learned 「やっぱり」, I understood it to mean "after all"--e.g. I'm afraid I can't come, after all.  I have to work.  

This website's explanation of  さすが suggests that it's a complimentary term that emphasizes the uniqueness of a person or situation, i.e. "Only you could have done that."

http://www.yesjapan.com/YJ6/question/3426/what_039-s_the_difference_between_yappari_and_sasuga

And this website offers a scenario in which さすが might used in a negative sense.
http://tangorin.com/forum/435/

You'll have to scroll down a bit to the English explanation, which comes after the French one.
The comment also mentions the casual version of   やっぱり, which is やっぱ.  I hear that a lot.