Hands down the best domestic airline- if they fly the route you are looking for book it. If they don't- consider flying somewhere else instead.
The planes- easily the nicest planes for domestic flights. Seats are roomier, nicer (leather) and each seat gets its own TV and video games- and this is in economy. Also you can order food and drinks to your seat through the touch screen- no need to ring the annoying call button. The planes are lighted inside like a disco in purple and neon pink. You can often upgrade to main cabin elite or first class for cheap the day of your flight if they aren't booked. There are also electrical outlets which are sweet on coast to coast flights.
The staff- are always super nice and generally quite attractive to boot. Even the people at the check in counter who usually at other airlines are pretty grumpy are always nice, efficient, and professional. I can't stress enough that this is lightyears ahead of other airlines like southwest/airtran, delta, american etc.
Prices- the kicker is you can often find flights cheaper than on other airlines which is mind boggling considering how much better Virgin is.