Prius is a great idea. My dad has had one (the older model, of course) for 2 years. Completely trouble-free, he gets 45 mpg and his wife gets >50 mpg on a 40-mile-each-way freeway commute (I guess he's more of a leadfoot).
I didn't know you could get in the carpool lane - that alone is reason to buy one!
I hear the new model Prius is even better. The Civic hybrid apparently doesn't have as good mileage.
The problem with diesels,
AFAIK, is emissions - other than needing to find diesel fuel of course.