It's funny, during the original run of the series I always cringed when the Ferengi were featured/highlighted. With subsequent rewatching of the show, however, I have come to really love these episodes and find them to be some of the most entertaining. Most of the credit goes to Shimmerman, as when they gave Quark something good to do he really was able to give consistently solid performances. "The House of Quark" jumps to mind as an example of an episode that could have been a shambles but that Shimmerman almost single-handedly turns into, IMHO, a great hour of television. Rom and Nog are less interesting compared to quark (although both characters got more complex with time), but still great to watch.
In "The Deep Space Nine Companion" it really comes across how strongly Shimmerman thought about Quark, his motives, aspirations, etc, and played him with more depth than most of the other actors put into their characters.