I have been trying to move away from TR, but haven't found anything that implements GTD as purely as it does. The problems I have with TR are the lack of a reliable mobile solution and the glacial pace of development. Some people don't like the rigidity of TR, but I like that it helps me work on one sort of task (capture, process, organize, do, review) at a time rather than random-tasking.
Another KEY feature of TR that I haven't seen in any other system is making templates and being able to have them auto-start. For example, my weekly review is a set of 13 steps I do in order, and when Friday rolls along, the Weekly Review project is activating and step 1 is the top action on my next actions list. That's going to be a difficult feature to live without.