myriad146 has done a great job of outlining the major issues involved in a VERY complicated situation. I'd only add a couple of points to his(?) excellent summary.
() The Google Task application is, ahem, less than stellar. However, to their credit Google provides for integration of the gmail with 3rd party applications. I use "RememberTheMilk," for example which is far more flexible and useful than the Google task app. There are other integration tools for task lists, as well. Just do some exploration of Gmail and see what suits you. Mine is set up to display and edit "RTM" from my online Gmail and the tasks show up in my google calendar on my Droid.
() If you're used to Outlook, you may find the Gmail threading of conversations to be problematic. After working with both for several years I find that Gmail is very good at consolidating some conversations but can be confusing and even cause you to miss an email if several people are contributing to an email topic.
One solution to this is to download the K-9 email application from the Market which presents emails separately (a la Outlook.) Just point it at your gmail account and you'll be able to use it either as a substitute or in addition to gmail. I use it as an addition (and I don't have it synch back to gmail.) That way I can manage my email mainly with gmail but I have another view of a long email thread that allows me to find a particular email when I need to. In short, I use it as a "bin" where my gmail collects. I just clear it out periodically to avoid accumulating a large volume of old email.
() Finally, I'll take the opportunity to gripe (once again) about one glaring weakness in the default "gmail" and "email" apps on the Droid. There is simply no way to password protect access to your email if you synch your email between an external account and your Droid. You can lock your phone, of course. But if you send/receive private or confidential email on any email account that you make viewable on your Droid, be aware that anyone who pushes the little icons will be able to see that email. This is a significant weakness in the Droid's handling of email and the major reason that many corporations will simply not allow the device to be used for confidential email.