My experiences may not be typical because I am a reviewer and so I sometimes get "special" treatment. Having said that, I find Cooltechnica hard to beat on customer service because Bruce is always available via ICQ (and this forum) and is prompt to return e-mails.
I have also ordered (as a customer) from Dangerden and Dtek with no issues and great satisfaction from their products, but neither site's customer service is as quick as Cooltechnica. Of course Cooltechnica doesn't have to mill blocks all day and night either
I have actually never had a bad experience with ANY watercooling store. I guess I was lucky never buying from Chip. All of the places seem to be run by honest hardworking people that are still very much tapped in to the community that they sell to. I would say just go with what is in your budget and that you like from any of the major stores.