I keep meaning to write a full wrap-up on this topic, but that will never happen. So this summary judgment for now:
Google's Calendar sync utilities are, in my experience, amazingly robust and dependable. There are three different places in which I enter or change calendar info, and on which I want to view it: In Outlook's calendar; online with the Google calendar; and on my Blackberry. Two Google sync apps -- this one for Outlook <-> Google Calendar, and this one for Blackberry <-> Google Calendar -- have been, for me, bulletproof. I enter, change, or delete appointments in any of the three locations, and the results show up correctly on the other two. This is a huge plus. Apparently there's a similar iPhone sync app, but I don't know how well it works.
Offline Gmail is, in my experience, far from bulletproof but still useful in a "better than nothing" way. Without this utility, if you weren't online you couldn't do anything whatsoever with the messages and info in your Gmail account. Now, if things go right, you can search and read messages while offline, like on an airplane -- and can write replies or new messages that will be sent the next time you log on. In its early version, offline Gmail seems to get confused easily about whether it's off- or on-line and to have other small glitches. For now I've disabled it except when I know I have a long airplane ride ahead. But that it works at all is something.
Bonus Gmail point: A new "panic-button" app, just released, which allows you to un-send a message within five seconds, if you have an instant case of sender's-remorse. I can think of times when this would have been useful.