I tend to try a lot of productivity apps and never really manage to stick to one.
When Process became available through MacHeist, I gave it a try but it didn't really work. I went back to iGTD that I had tried a long time ago. I wasn't super excited about THL when I saw the demo but I have tried it this morning and it's pretty cool... It looks very good! The keyboard shortcuts are well-thought.
One thing I think could be useful would be the ability to choose where to save the database. iGTD does that and that allowed me to have a synched DB between home and work with the help from Dropbox. With a coming iPhone app, I don't know how that would work out but that would be nice. (I know you can back-up and restore a DB but that's clearly not practical)
So, obviously, I can't say that I will stick to THL since I have only used it this morning but it looks promising and a lot more modern and clean than iGTD. (never tried Things, nor OmniFocus)
This is green