No, I can't say I have. I did send them an email asking about cell phone porting on sprint though. I believe I posted the email on this thread, though maybe not. Either way, they thoroughly answered my question with both a personal email and an informative link. Also they did this in like 2-3 hrs within my emailing them. I emailed in the morning around 10-12am EST. If they respond that fast and that thoroughly, and I have used them for tons of things and never heard of any issues with them, and when I google their cell phone sales and can't find a single complaint, and nothing compared to how wirefly is, then yes I consider them a good safe place to order from. However now knowing that walmart sells these phones I will likely try going to them on launch day to see if maybe they get the epic 4g.
Also I will say kudos to the ability wirefly has to respond to their customers. However I need to question them more so in lost returns, or lost shipments that are sent to consumers, as well as problems with them properly porting numbers and not allowing number switching. In googling I found tons of recent complaints about wirefly all posted within the last few months and all mentioning issues like this. Also they consistently say that the people on the phones either can't do anything, are rude, or tell them to go to their carrier to change the number or something, which leads to them getting the wirefly fee for changing things before 181 days. Both Wirefly and amazon may have good customer service, but when it comes to lost things in the mail or issues with phone numbers I would be far more likely to trust amazon.com to help with resolving the issue rather than telling people to pay $300 to resolve the issue.
Well, at least they carry the phones in-store as well (from what I've seen). The prices appear to be the same too. I saw the Captivate in-store the past couple of weekends and it was definitely at that $89 price with a new contract. 
