I have purchased more frags on line than I even care to think about right now. The trick is to make sure you go with a reputable dealer. There are some places you cannot go wrong with, like Live Aquaria as they give you a 14 day guarantee, there are also many sponsors of this site that do a great job. Some of them even have EBay auctions almost weekly.
I have only had one experience that did not start out the way I wanted. My wife decided to buy me some new corals and found an individual seller on EBay. His shipping practices left a lot to be desired, but eventually he made things right.
I can definitely recommend ordering from vendors who have actual stores. You may never have a problem ordering from an individul either, but I am a little more leery of that option.
we bought a lot off ebay after reading someone's good experience with Reef Empire. Their packing is AMAZING! and we highly recommend them. they also have a website and are a store in California. we also got some from reefgardener (eBay user name). they are a store in south florida. Their shipping wasn't as good, but everything got here fine and they even gave us an extra frag!