Well, for my $0.03 (inflation):
My wife and I own three cars. A 1992 Ford Explorer XLT, a 1999 Hyundai Accent L, and our new 2004 Toyota Prius. Before the Prius, it was just the Explorer and Accent. I owned the Accent when we met, and she owned the Explorer when we met. We both bought them because at the times, we could afford to buy them each outright. She had just been in a very serious accident, and felt safer in the Explorer, since her last car was a small car. So I understand her reasoning.
The Explorer gets about 10-15 mpg (got it up to 20mpg once, heading down the Rockies.) The Accent gets 30-35 (Long freeway drives get 40.) Well, my wife has a 9-year-old son that I became stepdad to, and both cars are fairly safe for him, in the back seat. And with a kid and a dog, the Explorer is GREAT for vacations. (We tried to camp with the Accent once, won't try that again.) Especially since we tend to go far on our vacations (we live in Portland, and our last three vacations were Yellowstone, Yosemite, and Olympic National Parks.) We drove the Accent to Tucson, without camping gear, and it was just barely big enough.
Well, now we've got a baby on the way (had a false labor a couple weeks ago, due any day now for real,) and decided that neither vehicle is 'safe enough'. I'd been wanting a Prius for awhile, and the many airbags, ABS, and VSC helped sell it over a couple alternatives.
But, we're still keeping the other two cars. I drive about 1500 miles a month, and my wife drives maybe 1000. By my math, switching from Accent/Explorer to Prius/Accent will save us about $100 in gas the first year. (At an average $2.00 a gallon.) Unfortunately, this does *NOT* make up for the $1000 a year extra it will cost us in insurance. (Although our second-runner-up new car would have cost $200 a year more in insurance than the Prius, with less gas savings.)
I'm not going to delude myself by thinking that the Prius will actually SAVE us money. I did a big fancy spreadsheet, and compared to our second-runner-up new car (A 2005 Hyundai Elantra GT,) it will take 10 years for the Prius to break even. (That's even assuming the Prius costs significantly less in maintenance.) And compared to just keeping our current two cars, it will take the Prius 20 years before it has 'paid for itself'. (Of course, that assumes that all three cars would survive 20 years.)
But, between the extra safety, the environmental factor, and the 'geek factor', the Prius is worth it. Will this mean I stop driving my SUV? No. We'll still take it camping every 3-4 months, and use it for the occasional haul of stuff to Goodwill or the dump, but it will no longer average 1000 miles a month plus 2000-3000 once or twice a year. Now it will be closer to 100 most months, with an extra 2000-3000 once a year. (I figure non-camping trips will be in the Prius, since it has more space than the Accent.)