The Derosa UD and Merak are very highly regarded bikes. Some people rate De Rosa higher than Colnago. I really don't think you can go wrong with a de Rosa.
I've never ridden one, but I've seen and touched a Merak. It is an amazingly beautiful piece of workmanship. The welds are ground so smooth that it looks like a single piece of aluminum. The fram could easily be built built up to sub-legal lightness.
very nice bikes. I don't know of anyone who has ever regretted owning a de Rosa.