Jeff, thanks for your recommendations. Do you know anything about Gary Fisher bikes? I had never heard of them, actually, until Justin read about them and showed me their stuff. My bike knowledge is pretty sad, however, since the only bike manufacturers I could name before we started looking around were Trek, Huffy and Schwinn.
(I was always the kid on the 10-speed or touring bike when I was growing up... never had a MTB.)
I'm leery of spending more money than we've budgeted for a bike, but we'll definitely be purchasing these in a store with guidance rather than online (the method we purchase most other items, such as technology, camera gear, etc.) Justin visited Bike World recently and came back with rave reviews of their staff, so that looks like a likely route.
Todd: Our reason for wanting to purchase mountain bikes is mostly for exercise and as an additional way to enjoy the outdoors and remain active. A little extra mobility provided by a bike would be nice on occasion, though I don't see us racing down a trail with our hair on fire, unless a cougar is chasing us.
It's the same reason I'd like to go horseback riding someday -- another way to see the outdoors. Horseback riders cover the terrain far faster than a hiker or backpacker can, but usually not as fast as a mountain biker would, so it's a happy medium... But a mountain bike's far cheaper and easier to maintain than a horse.
In regards to exercise, Jus and I are both in our 30's now and have finally realized we need to exercise more than we did as skinny twenty-somethings. That means more hiking and other activities. We have a wonderful rowing machine, but Justin and I both need variety in our exercise routine. Hiking's terrific, and our primary interest since it lets us observe wildlife, take photographs (another passion of ours) and be active outdoors. A mountain bike can offer the same opportunities -- even the wildlife and photography aspect, since mountain bikes stop occasionally, too.