switching from uml and code on the fly is so dificuld using pen and paper
Round-trip uml tools tend to generate crappy code and crappy diagrams. In attempting to make both easier and more automated they make everything more difficult. IMHO of course.
I'm feeling you, however thats a problem with the implication not with the consept. as jeff also points out, uml tools are costy and good uml tools, oi oi oi.
Use UML for design and specification purpose, only. As soon as you have a class with more than a few methods in it, the class diagram gets unreadable. If it comes to other diagram types like sequence diagrams for example, generated diagrams get totally useless. And since there is little value in class diagrams alone, rountrip engineering tends to be an expensive waste of time...
if your class has 'more then a few' (public) methodes(if there not public they don't (usually)belong in your uml),.... .....well more then a few, if your class has a lot of public methodes, perhaps you should evaluate your design.
round tripping with interaction digrams or generated interaction diagrams have never a fixed cirkel and changes should never be done by the developer (alone). generated Depence diagrams are like unittests.
Use UML for design and specification purpose, only.
thats actually interesting to repeat. as soon as your not building a liberie aren't you constantly re-designing re-evaluating or are you builing a factoryfactoryfoofactoryfactoryfactory, which can do anything it might have to do in the future? for small RFC, small programs I don't really think I'm getting my point across.