It is a great place to see lots of horses in person. Personally, you might get a better DEAL by shopping online and taking your time and comparing... but you can't see the horses in person and you have to pay shipping.
I would second being sure to try to buy the horse before it shows if you REALLY like it, because if it does well the price could go up or another buyer might beat you to it. On the other hand, if it doesn't do well, the price could decrease too!!!
All in all, GO to Nationals even if you don't buy anything. Pay CLOSE attention to the classes and what is winning... and check out those horses closely if you can. It's the only way to get a better eye, and try to get a show horse similar.
Good luck,
Andrea