I've also spent quite a bit of time, and effort over the years trying to find the perfect solution; something that is easy to enter information into, available everywhere, etc, etc....
For me, and for now, my mobile calendaring solution consists of a combination (not unlike others) of GCal and a Blackberry Curve utilizing Google's own Google Sync product for OTA syncing goodness. With the last few builds also including OTA syncing of Contacts, the deal got even sweeter.
I like to tinker with my 'system' at times, but I've found with my current set up, I don't have to tinker with it at all. I enter things either directly into the phone or on the Web, if that is available. (Actually, I use the phone many times with the computer right in front of me; it's so easy!!) I ALWAYS have my phone available, so that isn't a problem. My phone notifies me before an event occurs, and I also have Google e-mail me as well, just for good measure. If I feel I need to tweak the notification time (remind me 1 week before that doctor's appt so I can get everything together in time), I do that in GCAL at the time the event is created (requiring the web, of course).
I use GTD's method of day & time specific events, and day specific information, and it's worked out great so far.
But, as a previous post mentioned, if Google goes away, I'm up the creek! Or, yea, I get to build a whole NEW system....whichever way you want to look at it.
