Poseidon isn't everybody's favorite program, to put it mildly.  Try out
the Poseidon eval version and/or ArgoUML to be sure it's a good fit,
before spending money.  Also, see MagicDraw, which appears to share some
of the same ancestry.

It's nowhere near the same level of completion, and it's strictly a
linux-QT app, but I've *enjoyed* using Umbrello, which is not something
I can say for any other UML modelling tool, with the possible exception
of Visio.

In all my work, the UML diagram strictly for design documentation, and
the idea of automatically generating UML from code, or generating code
from UML, is honestly never a consideration.  ArgoUML does a reasonable
job of generating code, as does Umbrello.  

Also in the non-free department, Visual Paradigm has good integration
with Eclipse going for it.  "SDE", it's called.  

One thing that most of these modelling tools have in common, is that the
free and low-cost editions don't do reverse engineering, that is,
generate UML from code.  Poseidon does, in the "Standard
Edition" ($250).  ArgoUML, free, will import Java source and generate
UML too, worth a try, www.tigris.org