I'll be riding this year for the third year. When I did it for the first time, it was my longest ride by far, the previous longest being 105 miles. Now I'm deeply addicted to the long distance stuff!
When I did it my first year, it was really my introduction to long-distance cycling. I'd only ever done two centuries before it (TOSRV). The ride is basically dead flat, and you ought to be able to count on a tail wind in most years (although this was not the case last year). It's also aptly named; you're almost certain to get wet at some point on the ride. The ride is very, very well supported, although you will see a lot of people riding for time (and some who aren't) using personal SAGs.
As long as you keep the wheels rolling, it's really not that tough of a ride. You don't have to ride all that fast -- you've got 14 hours to do it -- you just have to keep moving. There are pacelines galore that form, so it's not tough to find folks who are your speed to fall in with and share the work. There are stops at 42, 60, 91, and 131 miles -- more or less. The 91-mile mark is the lunch stop, although there's food at the other three as well (including popsicles at the 131-mile mark, which were a huge hit).
If you aspire to a fast time, make sure you get off at the mass start with the lead group or you'll never see them again. The pace the racers set pulling away from the start was high-20s to 30+ (no kidding). I rode with the trailing group at a mere 25-26mph, picking up those who were shed off the lead group. Staying with fast groups all day, I finished with a rolling average of 19.7mph (and spent 1:20 off the bike at the 4 stops and fixing a flat at 75 miles in). More a testament to how flat Indiana is and how many fast tandems there were (thank you, thank you, all you fast tandems!) than a statement about my own skillz. Last year we had more of a tailwind and my on-bike average was 20+mph, and I'd ridden the ride backwards the previous day! You'd be surprised how fast you can ride when you're working with a group....
Looking forward to doing it again this year. Hope to see you out there.