Industry-leading fuel economy. Since April, 2008, when I bought the car, I have been averaging 4.9l/100km, the equivalent of 48 mpg US, 58 mpg IMP.
Consumer reports has rated the Prius as a leader in reliability.
The image of the Prius as being enviro-conscious fits in well with my own practices ("going green").
High quality materials, high quality workmanship--at least equalling that of my last automobile (a Mercedes C230).
I felt that the cost was reasonable given the technology. In addition, there was a total of $4000 in government tax rebates that lessened the cost of ownership a great deal.