What are the pros and cons of IMAP/CalDAV vs. Exchange for GMail and Google Calendar for iOS 4?
Exchange (Google Sync) for GMail lacks some features that you are probably used to--search and cloud drafts. The "known limitations" are listed here: http://www.google.com/support/mobile/bin/answer.py?hl=en&amp;answer=139635.

My experience with CalDAV in iCal are that multiple calendars on a single account are a bit squirrely to work with&mdash;you won't be able to get each calendar listed by itself under the primary google account heading. The calendars end up in a set of subfolders, depending on how you end up configuring it. That said, I don't know how the same thing would look on the iPhone.

What I do and recommend would be to continue using IMAP for GMail access--there's nothing wrong with it and you don't gain anything by moving to exchange. I do use Google Sync for contacts and calendars since it's the closest I can get to the functionality of MobileMe for free, and I'm very happy with it. For desktop access (aside from the webapp) I like BusyMac (post hyperlink limit reached; Google it) Software's products: one is an iCal superset clone and another is a sync utility that makes your Google Calendars show up in iCal just like MobileMe would.