TasksSimply put, there is no good sync for Tasks with OS X Mountain Lion. All of the material I've found indicates that there is no way to sync Reminders with Google Tasks. While the account set up says calendar and reminders it simply doesn't work for connecting to Google. Just use Google Tasks via the web browser and the awesome Android gTasks application on your Android Slate/Convertible or phone.
ContactsWhen I first set up the contacts sync I was only seeing 35 contacts. I knew this wasn't correct. After looking at the contacts section of GMail I figured out that it was only showing contacts that are tagged with "My Contacts". After adding the "My Contacts" tag to all of my contacts they all started syncing. However, you do lose all of your groups/tags in the contacts app on OSX. They are not removed from GMail.
I also discovered that a new contact that had been entered on the mac wasn't syncing out to the Google contacts. However, if I created the contact on Google contacts first it would sync out to the mac.
NotesNotes syncs with GMail notes, not Google Docs (now Drive). After setting up notes, I was experiencing super annoying pauses and loss of focus as the app would frequently sync with Google. At that time I was only syncing with Google for notes. When I turned on sync to iCloud in addition the pauses stopped. Oh, and before you turn off syncing, make sure you have the notes associated with that account saved somewhere else. Notes removes the notes you are no longer syncing!
After a couple days of using this set up, I made a horrifying discovery. Every one of the syncs that was being done with Google (every few keystrokes) was creating an entirely new mail document. I had dozens of emails which represented the progression of changes in the document! Also, in digging into the configuration, I wasn't really syncing with iCloud for the notes either. When I tried to turn this on through a different control panel it demands that you create an email account that will be associated with your iCloud account. No thanks. I am now not syncing notes either.
This article had some useful information on setting up notes sync.