Visual Paradigm Modeler is a really good low-cost UML modeling program. It only cost $99, which is a bargain compared to the more expensive programs such as Altova UModel. I haven't found any Open Source modeling software that is worth messing around with compared to Visual Paradigm Modeler.
It's very comprehensive, the editor is easy to use and it is very reliable. It has all of the features needed for modeling most software projects. It creates very nice object models, state charts and use case diagrams. For more advanced models, it supports a few other UML model types. Diagrams can be created with extensive detail, by filling in forms with about 2 dozen items. Everything is linked together, so it's easy to refactor just about anything.
If you need to do some modeling, try it free for 30 days, I'm sure you'll find it useful enough to buy a perpetual license for $99.