I have had on two instances where marinedepotlive sent me coral packs that had the leather coral already rotting once it arrived to my house. I called them immediately and they shipped me out a new one and told me to toss the dying one out. Other than that, my livestock from them all did well. Sometimes I can find things cheaper at my LFS other times I find it cheaper online.
The liveaquaria site is supported by Drs Foster and Smith. They are both great places to purchase items and Drs Foster and Smith also have a section to where they explain things about the hobby too.
I've used all the sites listed so far. Good stuff. I've ordered from this place before too: www.aquaticexchange.com, and they've always showed me fantastic customer service.