In the past week I've used Checkvist, Dynalist, Moo.Do, Flow-e, Workflowy, freaking Word documents, Nirvana... I'm going nuts but there's nothing as good as GTDNext, there's a few features you get so right they blow the competition out of the water.
@James please consider increasing the cost if it will improve the stability.
(and please prioritize offline support so that your server being down won't prevent us working in a browser with cached data! that alone would have negated the impact of this recent outage!)