I downloaded gtdtiddlywiki last week and my initial reaction has been that it's a fabulous program. I have been using it at work mainly as a todo list and also to manage my contact lists. Although, it's a great piece of software,I still think that Tomboy for Linux is overall better. Here a comparison on xx different areas between Tomboy and GTDTW:
Ease of use: Tomboy is far better with it super clean interface and ability to create wiki-like links.
Functionality: GTDTW, which is based on Tiddlywiki, is better right now. You can do many things, particularly with formating and tags is a very good feature. Tomboy has fewer functions and lacks tags. But, to add functions to GTDTW, you have to write macros. In Tomboy, you can write simple plugins. I think, the plugins might be a better options.
Future: I think both have a terrific future. Tomboy is likely to be integrated with Evolution, while GTWTD and Tiddlywiki are likely to be adopted by people looking for portability and even server side wiki.