Now, I know I should be using SVN to change the code, but I really don't have the time to install TortoiseSVN.
I found a bug that really annoyed me: I couldn't synchronize appointments which I already had in my Google Calendar but changed on my smart phone. After a close look at the code, I saw that Google.GData.Extensions.Who was not initialized.
I believe this bug should be fixed by Google, though. Below is a workaround. It would be nice if somebody who already has some SVN client installed would add this fix. Thanks.
From the frmMain.cs class in the SynchronizeLocalToGoogle() method:
foreach (Recipient recipient in appointment.Recipients)
Who participant = new Who();
participant.ValueString = recipient.Name;
participant.Email = recipient.Address;
participant.Rel = "self"; //Who.Rel was null -> Bug