I used to use Mean Repricer, which wasn't bad, but had a clunky interface. When they went through a patch of having glitches with csv imports, I looked around and decided to give RepricerExpress a trial.
The impact was immediate - we went from around 1450 Amazon Buy Boxes to about 1750 (based on about 4000 products listed). Additionally, the control panel gave me much better visibility on how our prices were doing, and the csv import/export process was very smooth.
There were a couple of things it took me some time to get my head around, but I'm a business-owner, not a techy person, so I suspect that was down to my personal limitations !
But what really made the difference was the support. I haven't had to use it too often, but each time i have, I've usually ended up talking with the same rep, Marie. She is unfailingly helpful, remembers our past conversations, is able to take the stress out of things (when calling support usually means I'm already stressed about something !), and usually volunteers additional helpful info as well as solving the original question.
There are a lot of companies who could learn from the support RepricerExpress gives.
If I had to sum it up, I'd just say "If Carlsberg did support...."