It used to be that they hired at GS-7, but accelerated you to GS-12 in 2.5 years. GS-9 at 6-months and then 11, 12 after a year each.
Within 10 years you'll get a capped GS-14 spot (105k-136k) and by 15 years, you'll have 5 weeks vacation, 13 holidays, alternative work schedule, 13 days of forever accruing sick time (think family medical leave for having kids). Oh, and you can't be fired unless you do something really stupid -- so clock in and out 9-5.
Oh and you get COLA increases even if you don't get step or grade raises.
I only lasted a year, but I have friends that are still there. The fact that you can't get fired also applies to the colleagues you'll be working with...