I secured a tee time here in late October of 2010 for both the Irish and the Straits course, and had the luck of playing the Irish Course as a single first thing in the morning. It is a surreal feeling teeing off ahead of everybody else as you hit into the misty morning sun.
The Irish isn't as demanding as the Straits so it is playable without a caddy and very easy to walk. Be aware of the flock of sheep that do roam both courses, although they are easily noticed by the bells they wear. My only caution on this course is when you play Devil's Elbow (#5) do it safely and don't try to hit over the sand, as it protects the green and makes it extremely difficult to land on the green on your second shot.
For the Straits course, I would definitely recommend a caddy as they know every nuance of the course. What club to play, where to aim, the slope of the greens, and of course the yardages. I certainly would not have enjoyed my round as much if I didn't have a caddy, so spend the money because it is worth the price.
If you are a golf aficionado and want to play the same course as the pros, make a trip to Whistling Straits and play the Straits. Yes it is pricey but it's not about the money, it's about the experience.