I mainly order from them because they do Paypal. Plus being a resident of California (where I believe Just Manga's headquarters are located as well) even though I have to pay a (minor) state tax.. the shipping's incredibly fast.. don't even have to bother with priority. It's usually at my house within 2 to 3 days of ordering. Since when I generally order from there (about twice a month) my order comes to over $40.00 (need over $39 for free shipping) it's a very win situation.
I mean I love Right Stuf the prices ARE better than Just Manga's but they usually take like a week and I wish they would use/accept Paypal as well.
Plus I confess I do like the buyer reward program. I'm at a fairly weak (though not that weak) 2,100 points currently.. I'm not sure what I want to save up points for.