For me it really depends on one thing--whether or not I've lathered up with sunscreen or not. I use scent to cover up anything that might turn fish off, such as gasoline, sunscreen, etc.
My cousin and I have fished together several times using the exact same bait, weight, line, and presentation, but one of us used scent and the other didn't. We found little to no difference in catch rate.
I will say this. The new scented Gulp baits from Berkley do make a huge difference. Don't know what's in those things but every fish out there thinks its food.
I typically use Spike-It because it really sticks and stays on the bait well. Just don't get the overspray on the boat--it sticks.