Nuevasync. I've had it for about 6 months now with no problem (knock on wood). Even though it costs $25 a year it is worth it too me. I tried the gmail push app and couldn't get it to work at all and researched Nuevasync and glad I did.
Google Sync (Exchange) has been running really well the past couple of weeks. Even over the holidays with lots of new iPhones. Taking mere seconds for emails to get pushed to my iPhone. Can't beat free.