Peyton Manning is the most bankable superstar in fantasy football; if you draft him, he will give you good numbers. Maybe, of course, you have reasons for not using a high pick or auction dollars on a quarterback. The difference between the 1st and 10th QB is smaller than the difference between the 1st and 10th RB. It's easy to get a cheap QB to produce 85-90% of the top QB's production, while it is more difficult (and usually requires luck) to find a cheap RB to produce 85-90% of the top RB's production.
To win a fantasy football championship, you'll need to take some risks. You're going to draft some players knowing they have the potential to bust. You're going to have to take some players for whom you more hope than know you'll get production. But you should also draft players that you know are going to keep your team productive from week to week, who at the end of the season will have provided you with outstanding fantasy numbers.
There's really only one major negative to drafting Peyton Manning: those brutal times of wondering when the Colts will rest starters, and counting on your backups during Week 17 (if your league goes that long). I keep telling myself that won't go on forever, that after something like six straight years of resting starters Week 17, someday the Colts will actually have to play 16 regular season games hard. Someday, I guess.