My only experience with them was when I bought 5 PS3 controllers and after I payed I realized they weren't official Sony brand... (it didn't state it anywhere, I just happen to notice the absence of the Sony logo) I immediately canceled the order, I didn't really want to sell knock offs (though they might be worth the low price?) and I was very nervous about getting a full refund. 5 days later the refund is on it's way back to via my paypal account.
So, while I can't vouch for their products - they don't seem to be scammers :) Good luck if you try them out, I'd love to hear of your experience!
I have heard from my close friends that they were using dealextreme.com for shopping for some time and they did not have any problems so far.
I have ordered few items last week from them but I am still waiting since items seem to be on the way now. I will let you know once i got it.