I have purchased countless items from Newegg, and find them to be very easy to deal with. I've bought cameras, lenses and almost 90% of my computer gear from them.
Their pricing isn't always the best, so it pays to double check against other vendors. For example, when I was in the market for the 100-400, they were several hundred $$ over all other retailers. Only after I personally called their marketing department did they even look into it. They lowered it, but didn't include the rebate from canon.
But on the 5dIII they were very competitive. If the 'open box' item is returnable, then I wouldn't worry too much about it if the price is good.
Have you checked out Canon's own website? They have a pretty extensive list of refurbs, but they sell out very quickly.
The nicest part of Newegg is that for most purchases over $500 you can get 1 year no interest. Just make sure you get everything paid off before the year timeframe.