Well, if nobody else wants to post a negative story, then I will.
I bought a project Tapper years ago. I actually got the board set up and running by cleaning all the edge connectors, pulling, verifying, and re-seating all of the EPROMs and burning a replacement for one of the EPROMS that had gone bad.
There were three problems I had left that I couldn't figure out. One was a vertical bar that would come up in the middle of the screen sometimes when it started up. Once it was booted with that, it wouldn't go away and it would make the game unplayable. The second thing was a loud buzzing (and no, you don't need to refer me to a thread about MCR buzzing, I've since learned more). And the third thing was an intermittent dropout of some of the music... for example, where you would expect to hear
doo doo doo doo doo doo doo doo doo doo doo doo doo doo doo
it would play
doo doo doo doo ___ doo doo doo ___ doo doo doo ___ ___ doo
So I called Eldorado (I think I spoke to John?) and described those three problems. He said to send the board set. He also said to send the power supply board. I replied that I had tested all the voltages and that they were fine, but he said that they needed the power supply for testing the PCB, so I said fine. I verified the price and asked him if I could just include the payment as a check in the shipping box and he said that would be fine.
I sent the board set and the power supply off and I included the check for the cost of the Tapper board set, as listed on the web site. A couple weeks later the board set comes back. I plug it into the cabinet and ALL THREE PROBLEMS are still there. Not one thing fixed.
So I called them (spoke to John again) and he says that they repaired it and it worked perfectly in their shop. I told him that I was still experiencing all three of the problems I sent it in for. So after going back and forth with him for awhile, finally he gets his repair guy to call me back. In talking with him I learn that there's a separate audio card in the cabinet. So I pulled that and cleaned all the connectors and stuff. Eventually, I got it so that the intermittent music problem would work sometimes and work not.
And the buzzing would happen sometimes and sometimes not.
The vertical bar down the middle issue never went away. But I did discover that if you turn it off and back on several times, eventually it will come up without the vertical bar problem. It's just some sort of random startup glitch.
So basically, they didn't fix anything and charged me $150 plus shipping for a few minutes of phone time with someone who helped me locate the external audio board and track down what the source of one of the problems was.
To add insult to injury, a couple weeks later they send me a bill for "repairing the power supply". This is completely ludicrous. I had thoroughly tested the power supply voltages before I sent it and they were fine. What's more, they system behaved EXACTLY THE SAME after I got the boards back as it did before I sent them off. EXACTLY. Not one change in behavior. So if they did any "repairs" to the power supply, they were totally unnecessary and made no difference.
I called to complain about them even daring to send me an additional bill when they didn't fix anything. John said that they had had to replace the Molex connector on the power supply. Funny, but it worked just fine before I sent it, and what's more I DIDN'T ASK them to do anything to the power supply and they never REQUESTED whether they wanted me to "repair" the power supply. I refused to pay the bill, and John said that they'd never do business with me again.
But why I would want to send another board set to a company that charged me money and did nothing, I can't imagine.
Note that I still have the Tapper, and it still exhibits all three of the symptoms that Eldorado claims to have fixed. Like I said, I have a workaround for the vertical bar issue, most of the time I can keep the buzzing pretty low, and sometimes if I futz around with the harness and the sound board I can get the music to play all the way, but most of the time not.
In retrospect, I wish I had shot video before I shipped the game and right after I got the boards back, so I could have documented the monumental fail non-fix by Eldorado better.
For a lot of stuff, they are your only choice for getting boards repaired, and I'm sure some people have had luck with them successfully repairing things. But for me, they didn't fix my problem, charged me anyway, and tried to charge me for something I didn't ask for (and that wasn't needed).
That's just my personal experience with them.