I consider Poseidon a useless tool. You could waste hours fiddling with it when you could be accomplishing something. If NetBeans is worse then I will never use it.
It is worse in the way that you have to go to the palette all the time to act on your graph, while Poseidon has a better GUI. That's all. Other than that, i find it better as it has much more powerful options.
If they have managed to find a way to keep all the trivial classes that you don't want ot know about out of the way and off the diagrams.. in some way that doesn't require hours of fiddling.. and if the UML & source are always in sync with out ever having to press even a single key to sync them... and if it gets to the point that I could write almost my entire program by drawing the UML then fleshing out some methods in the source editor.... only then will UML be worth anything at all to me. Otherwise it is a distraction that simply sucks away time I could be using to get work done. On much larger projects with much larger teams working on the same code, things could work differently... but so far I have never seen a UML tool that wasn't a significantly negative factor in terms of doing any real development... pen and paper have been far more efficient thus far. (And to think some companies think we should pay for that decreased productivity
You put the classes you want in your diagrams. So if there are classes you don't want to see there, just remove them from the diagram (but not from the model)
Everything is always synched, which is why it sometimes lags a bit.
You can make much everything in the diagram, then fill the methods. I did it and it works fine.
I don't agree with your vision of the usefulness of ULM. when you are designing something with more people than you alone, 'fiddling' an uml graph is much faster and cleaner than using paper and eraser. Moreover, once everyone agrees, you can immediatly start coding as all classes are ready to be filled. Also, explaining a lib or the architecture of your code is much easier with an UML graph than with code or abstract explaination. But as always, YMMV...