I prefer to do my own plans and periodization (long-term plan, about a year at a time). Unfortunately, Garmin Connect lacks this completely.
There is some planning functionality for dragging workouts to the calendar in weekly view, but nothing that shows me a summary of the coming weeks so I can see the planned (and performed) progress. And the year view of the calendar doesn't actually tell me anything at all (really, does anyone use it?).
I've tried the TrainingPeaks integration, but TrainingPeaks is way too restricted to a particular training methodology for me. I can't plan weeks by mileage, I can't create workouts based on HR zones, the ramp-up before races is too weak for me so I have to adjust everything manually, there is a lot of stuff (restrictors/week types/etc) that is not explained _anywhere_ (I guess they want you to read their books) etc...
Please, Garmin, add this to Garmin Connect. We control freaks will love it. 
And in the meantime, does anyone know of any 3rd party service that will let me do long-term planning (and preferably - but not necessarily - monitor how my activities compare to the plan)?