Now that we're in the blessed era of iPhone ActiveSync to Google Calendar and Contacts I've been comparing Syncit for Outlook 2007 ($10) to Google Outlook Calendar Sync (free).
Both of these products have had serious bugs in past experiments. gSyncIt has the big advantage that it will also sync Outlook Contacts, Google's current Outlook Sync only does Calendar items. gSyncIt also has many more customization options.
On the other hand, gSyncIt does not do meeting attendees.
Neither can handle everything one can put in an Outlook meeting of course. Attachments? Puh-lease. For this reason alone all sync must be unidirectional (either way), bidirectional sync of this sort will only end in tears. (iPhone to Google Calendar, OTOH, is much simpler and mostly works.)
I'm thinking. I want to see how the Contacts goes, and I might hunt around for a third solution ...
Update 2/11/09: The gSyncIt Add-on is definitely causing Outlook 2007 outlook.exe process hangs. Not necessarily their fault; Microsoft's Add-ons have the same problem. The Outlook 2007 Add-on architecture makes DOS 3.1 TSRs look good. I've had to uninstall gSyncIt. I suspect the only safe way to get Outlook contacts to Gmail will be to periodically wipe them from Gmail, then export from Outlook and import into Gmail. Annoying!
Update 3/17/09: Google Calendar Sync was working until recently. It started giving me error code and nonsensical "connect to internet" messages. I had to delete my Google calendar data and redo the sync to make it work; nothing related to the error messages. Fortunately I only do 1 way sync!