Online calenders intrigue me, but little else. I can see the value of adding events and reminders and the like, but ultimately have never felt drawn towards using them. However, I do occasionally use the calendar feature on my iPhone, which works pretty much identically to any online calender app, but with the additional benefit of being in my pocket (accessibility at its finest). As a student, I have a short list of important dates and appointments to keep track of, but feel very certain that things will change dramatically when I'm teaching. When that day comes, I think I'll rely on GoogleCalendar to keep my schedule straight.
I went and signed up at Rememberthemilk.com, and checked it out. I've heard the name tossed around in conversations recently, and saw this opportunity as a good excuse to see what the app is all about. Basically, I was unimpressed. Deeper contemplation on the issue leads me to feel that maybe our society is becoming over-organized, to the point of becoming cluttered (albeit in a more structured fashion). At Rememberthemilk, you can make lists, organize lists under different tabs, share lists with others, etc. In my mind, using this type of app is just another thing to manage, beyond the items that you had to take the time to list in your head, type up in the app, organize, and check off. Whew! I'd be better off with an old pen-and-paper honey-do list. The one thing a productivity app does have going for it is its medium; keeping lists and events online does make them harder to lose.
Recommendations? Google. Use it for everything. Especially if you're not so tech-savvy (like myself). They'll do all the heavy lifting for you (like Hamburger Helper!).