The key feature I used Astrid for was the ability to have recurring tasks based on the date of last completion instead of fixed intervals. Does anything else out there do that?
When: http://when-app.com Doable: http://doableapp.com
When "rips off" Clear's UI and looks interesting. Doable has a very custom UI and looks like it has a couple more features, e.g. statistics. It looks like neither supports date-fixed tasks. If you need both you'll likely have to use Reminders or something in addition to one of these.