UML, or any other sort of high-level design planning, is a bit against the hacker spirit so my guess is you wont find many here who use it. Hacking is more bottom up, start with something simple and just keep iterating it into something larger.
The only instances I've seen UML being useful is in very large software projects with many people involved. Having an abstract way to convey program structure can be useful there. But for anything less it will probably slow you down and restrict your freedom, IMO.
I find I need some sort of plan for all but the simplest of projects. I've even found it valuable to return to the diagrams (class only, the others don't work for me) to try to remember what the heck I did a few months back.
The only instances I've seen UML being useful is in very large software projects with many people involved. Having an abstract way to convey program structure can be useful there. But for anything less it will probably slow you down and restrict your freedom, IMO.