I have ordered a few times from SWF.com. I had a problem once with a fish that was improperly packaged. It took awhile to get resolved, but it did. Other than that incident I have been impressed.
As far as Petcos go, I have purchased some very nice fish from the one in my area. I normally support the family owned LFS, but sometimes ***** has some deals I can't pass up.
I bought two 6" sohal tangs for $50 each. (No they aren't in the same tank). They have been alive and healthy for several months now.
I bought a nice marine betta for $60.
My most recent accusition from ***** was the biggest purple tang I have ever seen for $60. He is easily worth twice that. I got him through a mild case of Ich and now he is doing great.