I second the notion of proxy bidding always ending up much, much higher than you think. Let me give you an example.
I recently bought Winds of Thunder for 1680 via SMJ which amounts to a raw price of about 20. A bargain, right? Copies of this game usually go for 80-90.
But after the shipping cost from seller to the SMJ HQ in Japan, SMJ commission and the fee for paying the seller the cost is already pushed up to about 40, and that is just the beginning. After that you have to pay shipping for the item to be batch sent to SMJ HQ overseas plus shipping from there to your residence.
At the end when all is said and done you are lucky if you saved 10 overall and you got the item several weeks later than you could have if you just went the ebay route.
Of course if you buy a big batch of things then the final shipping cost will be reduced per item, but overall you are not going to end anywhere near that very tempting low Yahoo.co.jp price that wet you appetite in the first place.
SMJ is useful under certain special circumstances though.