I've bought twice from SL Hardwoods, with 2 quite different experiences.
First time was 1" American Black Walnut and was fine. I used it in a project for F&C and mentioned that SL were good in the article.
Second time was 1" and 2" American White Oak. The 1" was OK, but the 2" was appalling - seriously bowed and with some massive defects and dead knots. I had to make a lot of fuss before they agreed to change it, I think had I not mentioned that it was for another magazine article I'd have been stuck with it. To be fair though, in the end they gave me a replacement free board that was OK, so worked out fine in the end.
What I take from these 2 experiences is that you will probably be OK with mail order, but it depends who picks the boards. I suspect that a lot of times they will just send the first board off the pile, whatever it looks like. Unfortunately you cannot rely on the fact that the timber will be selected with the care you would use yourself.
Mail order is OK if you have no other choice, but in view of the postage costs and the uncertainty I intend to avoid it where I can.
Hope that helps, Ed