Spirit just started from LAX to LAS and it was so cheap I had to try it. Here's what I learned:
You are charged for checking in bags.
You are charged for carrying on a bag unless it fits in the chair in front of you.
If you bring two bags on board you will be charged for at least one.
They weren't actually too good about checking whether or not to charge you. Although I didn't pay to use the overhead no one seemed to say anything when I did.
Baby strollers and wheelchairs are free.
If you have a baby, you get to bring a baby bag on board for free.
Since most don't have much on them, the boarding process is like 5 minutes. You'll sit on the airplane bored for 25 minutes before wheels up.
At least at LAX, there aren't really any Spirit employees. A random airport agent showed up a couple minutes before boarding.
LAS airport has plenty of real Spirit employees.
If two people are traveling together, don't bother paying the extra money to sit together. My companion and I didn't pay but were issued to seats together.
Yup, learned all that on one round trip flight.