I used to work at the NASTAR course at Breckenridge and really liked it. It's a great program for all ages. Some people are WAY into it to the point of being obsessed. However, for most recreational skiers it's a great thing. If you've never raced before, it can be a real ego deflater to realize just how hard running gates can be. It'll also make you appreciate ski racing a lot more.
Generally, NASTAR costs $5 - $15 depending on what the ski area wants to charge . Your ski area may offer an "Unlimited" pass that lets you run the course all day long, or it may not. Usually there's two sets of gates set up so you can race your friend. How a NASTAR course is set up and run can be quite different from resort to resort, and that's part of the fun.
NASTAR has a neat handicapping system. It's quite complicated, but if you read the documentation it seems to make sense. In fact, if you're one of the aforementioned obsessed people (any you know who you are...) then I'd suggest asking the NASTAR people at your course if you can get a copy of the documentation that comes with the NASTAR software - it's all spelled out in there. Ultimately what you get after doing two runs is a "handicap" number. Your number will probably be something like 30 or 40. What that means is, you're 30% or 40% slower than the pacesetter on the US Olympic team. (Hence, ego deflation.)
How is that actually figured out though? Well, at the beginning of a ski season someone at your resort gets to race against former US ski team member who, in turn, have raced against someone on the US Olympic team. I'm not sure how many runs they get to do, but it's enough to figure how they compare to the US Olympic team. Generally, they'll get a handicap ranging from 8 - 15. In turn, more people at your resort race against this pacesetter and get their own handicap. This group of people generally runs your NASTAR course. Each day they set it up, and each day they're the first ones down it to set the pace for the day.
At that point, things get a little complicated. Since we know how your pacesetter compares to the Olympian, we can figure out what time the Olympian would theoretically have had. When each individual skier runs the course the time can be directly compared. Then different factors are figured in to actually get your handicap score - such as how old you are, if you're skiing, snowboarding, telemarking, etc, etc. When you initially register for the day, you should be able to get a unique identifier that will let you log into the NASTAR website and see all the times for all the people on the day you competed. It's a statistics junkies dream. (Not getting that identifier is a good sign the people at your resort have no clue how to use the NASTAR system.)
Of course, all this probably falls apart regularly because I suspect a lot of ski areas don't have pacesetters set up correctly or don't know how to use the NASTAR software right. Plus, your local pacesetter may have an awesome day and set the bar on the course way too high. Or, your pacesetter may show up hungover and not perform as well as they normally would. So, take it with a grain of salt and remember it's recreational racing.
Now, one thing I thought was awesome at Breck was they set up a boarder-cross/skier-cross course and people could run it for $1. It was a killer idea and I'm shocked it never caught on. I had a blast running that on tele's. We used to let 4 people run it at once, so you could really have a lot of fun bashing into your friends. I think they stopped having that though and reverted back to just a traditional NASTAR.