I've worked a bit with UML 4 to 5 years ago, and quite frankly, it confuses me more than it helps : I find too often that the nice UML blue-print doesn't fit well actual data, or workflow, and has to be trashed after the development has already advanced quite far. Maybe that's just me...
Proper datamodeling is very important though, but I prefer thinking it directly in Perl, or relational databases, or whatever else I'll be using to manage the actual data.