A brief outline of my experiences so far:

OmniFocus and iGTD

• extremely similar appearance and workflow
• work well
• very quick to enter info
• both somewhat sloppy with the GTD model
• neither provides a proper Waiting For list

Midnight Inbox

• looks great - brilliant in fact
• has a somewhat in-your-face attitude towards GTD
• seems quite rigid, which may be a good thing or not
• some interface weirdness
• I found it slow to use, because it is often not clear how to do something
• should provide you with a smooth workflow, but somehow doesn’t
• doesn’t respect date format preferences - insists upon a North American style date entry
• despite its GTD rigidity, doesn't handle Waiting For items properly
• excellent Review view, let down by interface behaviour

Things

• still in alpha
• very calm, soothing interface
• takes the opposite approach to Inbox, leaving management and overview to the user
• doesn’t impose many structures
• very simple to pick up and start using
• doesn't yet but will support Waiting For

Daniele