In all honesty, I haven't made it to a U.S. Open Qualifier since I've gotten my handicap down below the cutoff because it has fallen on a date that I wasn't able to make it. You better believe I played in all of the other USGA Qualifiers that I could play in, and would recommend everyone that is eligible to do the same.
Some may say that you aren't ready, but the only way to be ready is to play in your first one. Then you will have a taste of what the competition is like and, if you're like me, you get the feeling that you
can
play with these guys...at least at the local qualifying level.
Another thing to keep in mind is that it's two rounds in one day, at least it is here in Northern Nevada. That really takes a bit of a toll in the middle of the summer here in the high desert, so it really takes preparation and dedication.
I would also echo the sentiment that you should play in your local golf associations bigger tournaments like the Mid-Amateur (if you're of age, of course) to get a feeling of real competition...not your clubs competition.
Basically what I'm saying is that playing and getting experience in these things is the best thing you can do. Plus, it's the best way to play some local course that is in excellent condition for cheap!!