I target specifically Eclipse users. There are many advantages of having the tool integrated into the IDE: familiar and customizable interface (views, window system), use the same resource management (files, projects), version control.

Feature-wise it's more difficult to differentiate but WireframeSketcher has some distinct features like master screens and storyboarding. There are more subtle differences like the possibility to insert icons in any piece of text which gives a lot of flexibility. Take a look at this blog entry: http://tinyurl.com/yeuo3v4

Speedwise I'd say WireframeSketcher is as close to native speed as it gets considering that it's also cross-platform.