The thing that sucked me into using Google Sync started with the calendar.  I wanted to use what was available without installing a whole other application that was constantly running.  I am constantly adding or removing things from my phone or desktop and keeping both the same was a pain.  Using Google Sync whenever you make a change, on either the device or the web, the calendar will stay in sync.  Now you don’t have to remember on which device you made the change and manually sync with the other.  The other part involved in Google Sync was the ability to sync your contacts.  I didn’t jump on this right away, but once I gave it a chance I was hooked.  I absolutely hate having to input all my contacts in when I switch phones or when I wipe out a phone.  Using Google Sync, when you get a new phone or wipe an old phone out all you have to set up is the Google Sync settings and your calendar and contacts are back to normal.  No more having to put them in one by one and you don’t have to worry if you switch to another carrier, as long as your phone works with Google Sync you can recover your calendar and contacts.  Another big thing is you can input contacts on a full sized keyboard within Google Mail on your desktop or laptop and those new contacts will automatically sync with your phone.  It is ten times easier to sort through information, update information, and remove information using the web interface of Google Mail than trying to fumble through the process on your phone.  

I would definitely recommend that you use Google Sync if you have a compatible cell phone and a data plan.