OK, so here's my full report.
I used a company called abidko. It was the first I found and when I contacted them, I didn't know about all the others mentioned above. The name sounds similar to akibado, but they are not the same. Abidko is based in Singapore and they have people in Japan who bid, pay and ship on their behalf.
Short version - Overall, I have to say my experience with them was great. Yes, it was a bit pricey, but as GP mentioned, they did do quite a bit of legwork and communication for me (see longer version) and I don't think it was more than the other companies.
Longer version: I wanted to get ahold of some concert tickets and Yahoo Auctions Japan is pretty much the only way I knew of. I found an ad for abidko somewhere on this site and sent an e-mail. Response was quick and polite so I did some research (found they had a high feedback rating on yahoo auctions) and proceeded.
I have to say, the guy I dealt with was great. I placed my bid through the site and sent my deposit via paypal. I looked at other auctions and based my maximum bid price on the price range of similar tickets. The range of prices was a bit wider than I expected, so I figured I'd place a fairly high bid in case bidding got hot and heavy when I wasn't around. After all, the way auctions work is it won't go to your maximum bid unless someone is bidding against you, right???
What I didn't count on was a problem with shill bidders. This had nothing to do with abidko. It turns out that there's this flaw/feature with yahoo's bidding system - something I've never had a problem with on ebay. Someone comes in and bids very high. Then, they cancel their bid. Instead of your bid going back down to the price it was (ie. 500 yen more than the previous high bid), it stays at your maximum bid. Moreover, when they cancel their bid, there is no indication in the bid history that their bid even existed. Evidently, this practice is commonplace for expensive items (or at least for the tickets on which I was bidding).
I actually lost the auction to someone else, but they cancelled their bid after the auction was over leaving me the winner at almost 10,000 yen MORE than the previous bidder. As I looked at more auctions, I realized this was going on constantly and I should have taken this into consideration before I placed my bid. As an example, there was one set of tickets I saw for over 100,000 yen. A few hours later, those same tickets were back down to 34,500. So, in many ways, I was basing my original bid on some artificially inflated auctions to begin with. My mistake, yes. But I wasn't about to pay 9,500 yen more than what I technically would have won the auction for (based on the next highest bid below mine).
The guy at abidko was great. Since I technically didn't win the auction (the person who cancelled their bid after the auction ended won it), I wasn't locked into buying the item. So, I offered the seller what I should have won the item for. The seller refused and was going to re-list the tickets. After thinking about it some more (and realizing that the auction I had won seemed to have jacked up prices all around since I didn't cancel my bid. LOL) I decided to split the difference with the seller and offer 5,000 yen more than what I would have won them for. This seemed fair to me based on current auctions (I now know how to tell which prices have been inflated and which weren't). The seller agreed.
All this communication I did in English to abidko and they communicated with the seller in Japanese for me. I'm sure this was more work than they had planned but they were always polite and customer service-oriented.
From there, everything went smoothly. There was a little sticker shock as my invoice came in Singapore dollars. But, paypal converted it to USD and it was just what I had expected.
It took a couple weeks for the item to get sent from Japan, but I expect that was a delay on the part of the seller and/or a slow mail system (the seller was in Kyuusyuu, the agent was somewhere in
Hokkaido).
I do plan on purchasing some other items on auction at some point in the future and I will use abidko again. However, I will be more savvy about my max bid.
For anyone interested in getting something from Yahoo Auctions, I would recommend abidko. Check the prices of any company you use so you know exactly what the overhead and fees are. You want to make sure you know what you're going to pay. And, if the item is fairly expensive, I would highly suggest doing plenty of research on the auctions before you decide on a bid price - watch similar items, bookmark them and look at what they actually sold for and determine whether the price was inflated by shill bidders or not.
Long post, but I hope it's helpful.