Have a look at Trace Modeler
Have a look at Trace Modeler, it's a UML tool that's actually easy to use and smart about its domain - sequence diagrams.
It even has two refactorings, split activation and inline call ;o)
Plus it does all layout automatically so you can focus on the content.
It's really fast for editing, simply drag & drop an activation and it will reconnect and move everything around it so the diagram remains correct and pretty.
There's a 30 sec demo that shows you how it works at
IMHO, most uml tool vendors focus too much on providing an exhaustive features set and forget all about basic usability.
That's why I created Trace Modeler. I does only sequence diagrams, but it does it very good.