Thanks for all of your replies and i have lots of useful info for newbie and second photog. I manage to get a nice shot at reception even my flash were failing me I will share once i got a green light from the bride.
Here is some bad experiences first:
1. Once bride introduce me to the pro and tell her I am backup, tension is being created, she said she never need a backup. i think one major factor is that 70-200 2.8 IS lens(rented) that is causing the trouble because it really make me looks like a pro. I paired it with a gripped 60D and this thing is good for working out, not taking pictures pro-long. My left bicep is almost swelling after 2 hours of shoot.
2. Since tension is created, she told her assistance to block my view whenever they can, geeze this is real bad if you ask me because I hate ppl following me. you might think I am paranoid but I am not, I on pusrposely walked to a dead corner and pretend to shoot there(actually resting) and I see him coming to that position right after i went back to shoot, sigh.
1. I stayed away from the pro for sure so I won't be in her picture.
2. 70-200 2.8 IS is reallly really fast in focusing even with a 60D, I assume if you have a better body you can really make use of those outer focus point, I stay centered all the time. I shoot without a flash the whole ceremony as it is too darn heavy to put another 580 on my 60D+this lens combo. For those who is using 70-200 F4 IS and thinking of upgrading, make sure you rent or try one before doing so as you might not like it due to its weight. I tried to remove my grip to reduce weight but again, it felt I cannot balance the combo out so I have to put back on right the way.
3. FF or clean iso performer is a must, when I compared files at 3200 and 1600 between a 60D and a 5Dc, 5dc rocks out, with a little of PP the noise is either gone or become a nice noise that ppl might think you did it on purposely.
4. for a crop body, no question asked, you need 17-55 and a flash if you are planning to shoot wedding.
Mistakes that I make:
1. I trust canon ETTL system via infra-red = failure, they did not see each other and never flashed my external flash.
2. Should charge more, at first I think i only shoot for a couple hours during ceremony and reception, but when I add up all this extra expenses(rental/travel/etc....) and i think the whole job casued more hours than what i expected.
3. I didn't work out before using the 70-200 2.8, if you are not a strong guy or gal, make sure you either rent a mono-pod or work out extensively just for using this lens.
4. I changed the shutter speed accidently and didn't realize it until going home, some of my group shots turns out with little blur as I was shooting at 1/10 with a flash, how stupid I was.
1. the pro girl use just a 580exII bounced up for group shots(4-5 rows and 15-16 ppl each row), is this enough power? I assume one will have to let some ambient in, right? If yes, how to fill the shadows on the ppl's face as there is strong spot light coming down from the ceiling wor?
If you are new to wedding, feel free to ask me any questions although I am also new as well.