I first heared about it when I read a a showcase paper about the integration of Google Gears. Recently it seems to get more and more popular. In my eyes this is mainly due to the lot of different integrations they offer. People probably wouldn't want to go to the Remember-the-milk homepage for entering their tasks every time something comes them in mind. The integrations into email, calendar and even mobile phone is what makes it interesting. When I'm looking at my Calendar I can enter a task and I'll see it in my email account at the same time.
The "disadvantage"? It is havily based on Google services, so it uses integration into Google Calendar and Gmail, so it may be a disadvantage for those who don't use them. I personally can just recommend it.
From a technical point of view, RTM uses a lot of AJAX features. The integration into Gmail, which has to be installed as Firefox Plugin is really impressing. Just from the ways it integrates itself by making use of Gmail's function for showing messages to the user or the way it observs email labels for possibly integrating them directly as tasks.