Although I have no experience of Model Slipway, Caldercraft and the rest, I am most of the way through building a Dumas kit -
http://www.modelboatmayhem.co.uk/forum/index.php/topic,46157.0.html . No balsa in this one, and some good quality birch ply and mahogany planking. However, as I mention in my build log, the plywood supplied for the frames is very poor quality and the fittings, although chromed as befits the boat type, are poorly cast and not cleaned up before chroming.
The overall design of the kit is good - it goes together well and builds into a fair and strong hull - but the instructions fall short in a few areas. As with some other kits, a firm eye on price/profit has, I imagine, led Dumas to provide the cheapest materials they can get away with and there are precious few signs of any quality control. Replacing the wood with better quality materials is perfectly possible - but given the price of these kits, is that a reasonable solution?
Overall, I probably wouldn't go for another Dumas kit. From reading around, there are better quality products out there. But if they are the only source of the boat you want, they provide a sound base for a good model with a little extra effort.
Hope that helps!
Greg