I liked the Cerveny BBb tuba that I owned, but the quality/workmanship varies greatly among different samples of the same model. Certain models are generally better than others too. I had a 786 stencil (the rose brass version of the 686). I thought it was a great sounding horn, and liked the way it played, but decided that it took too much air for me.
The brass in their horns is generally softer/thinner than others and is more easily dented. This shouldn't be a problem if you take care of it. For the money I think that they are veru reasonable horns, but you could probably get better horns for the same price used. The Miraphone 184 that someone is selling on this board is one that comes to mind as an alternative worth considering.