I have Google Sync enabled so I can sync my contacts and calendar with my iPhone. It makes it easy to maintain my contacts in Gmail and have them sync instantly with my phone without a cable. It's also great having the peace of mind that if my phone breaks or is lost, all my contacts are backed up. My one issue is that I have to carefully choose which contacts I save because when I clutter up my Google contacts it makes my phone hard to use.
Anytime you email someone, they get added to the All Contacts group. If you email them often, and they aren't in your My Contacts group, they get added to your Suggested Contacts group. If you add or edit a field in a contact or add it to a group, Google will add it to your My Contacts group and begin to sync it with your mobile device.
You can tell if a contact is in your My Contacts group in two ways. Obviously you can click on your My Contacts group and scroll the list of contacts to find it but most people have a lot of contacts and this can be a pain. If you search for a contact or click on a group to find it, you can tell if it's in your My Contacts group if it's listed after Remove from... in the Groups drop down list.
That is the trick; you can remove a contact from your My Contacts group after you've cleaned up the name, added some more information or added it to a group. Keeping it out of your My Contacts group will prevent it from syncing.
Why is this useful you ask? There are lots of reason you might not want to sync some contacts. Old contacts you want to keep for references but don't need to contact often like past landlords or employers. Exes or international contacts that you rarely call. You can import an email roster for a group you are in and put them in their own group but not your My Contacts group. On the iPhone, and possibly other Exchange ActiveSync (what Google sync is based on) implementations, you can click the groups button on the top left of the contacts screen to search the Gmail Sync Global Address List which includes all your contacts. In a pinch you can always browse to the Google Contacts page to see them all or use the mobile Gmail where auto complete still works for all contacts.
This trick is also a great way to add birthdays to your Google Calendar if you have the contacts birthday calendar turned on. I use it to add young relatives or kids of my friends that don't have their own contact information yet. Make a new contact with just a name and birthday then remove from your My Contacts group. Then you also have a place holder to enter contact information when you get it.
One caveat is that everytime you edit a contact or put it in a group, Google will re-add it to your My Contacts group. A small inconvenience compared to the flexibility this adds.