Remember The Milk is a great task list manager, with excellent sharing features and a powerful query facility. But the UI could use some improvement. A Bit Better RTM goes a long way towards closing this gap. Very thoughtfully designed, and it's truly impressive what the developer has been able to accomplish with CSS and scripting!
One issue I would like to report with Ctrl-M: It allows moving tasks to Smart Lists, as well as regular task lists. The task will be stored locally, showing the smart list as its assigned list, but it's not visible when the smart list is displayed. When I subsequently refresh the browser, causing a re-sync with RTM, the task disappears. I have made this mistake a few times.
Please note that I am using the chrome plugin, not the firefox version.