I am flying alone with my lo for the 1st time this month. He is 13mos. I was planning to buy a seat for him and take his carseat. However, several of my friends said it's best to carry him now, which I know I can do until he's 2.

Southwest recommends the car seat for kids under 40, but allows you to hold them as long as you complete a boarding verification form (which is for liability reasons, I'm sure).

Kristi and others who have flown a lot with your lo's, what do you think?

HA---under 40 lbs, not the age of 40.

The American Academy of Pediatrics recommends against lap flying (see link below). I have always thought the idea of flying with a baby in your lap was contrary to all the safety suggestions we've had drilled into our brains as parents. There's no way that would be safe in turbulance or any other sort of emergency. But, that said, the airlines allow it, and I know lots of people do it, primarily to save money I think.
http://www.aap.org/advocacy/releases/travelsafetytips.cfm

I've never bought Jack a seat and he's flown many times in his life. It's saved me a lot of money to take him as a "lap child." I have never felt he was in danger. I hold onto him tightly during the take off and landing. And I hold onto him throughout the flight too.
With today's prices, I just wouldn't be able to fly home to see my family as much if I had to pay for Jack too. This trip I'm on now cost me like $400 and paying for Jack it would have been $800. Yikes.
Ya know, if you fly Southwest and the plane isn't full they will let you use the seat next to you for your LO for free. Not sure if you can check your car seat at the gate if there is no extra space though. You can check a stroller so why not a car seat? But I'm sure it's not something they'd encourage if you called and asked about it. I feel like someone told me they did that once...
When we flew this week the planes were not full.
It's a personal call though.
And yes you'll need his birth certificate to prove he's under 2 when you check in.

Alex flew tons before he was 2. I always held him in my lap. I sometimes wanted to have him in a seat, so I could be hands free, as I had my oldest to entertain as well, he was 3 and 4 at the time. But it never was a problem, I never felt like he was gonna fall or anything like that. And like Kristi said, if the flight is not full, you can use the empty seats.
Def. take his bc, they usually don't check us until its almost their bday, but they have once or twice.