From my personal experience, I love this service. I have two Civic hybrids parked outside the building I work in downtown. A minicooper, solara convertible and Tacoma are just 3 blocks walk. I use the Civics 4-8 times a month for about 2 hours each trip. I've taken the Mini out once, you may have read my review on that.
From a car enthusiast's perspective this can also be a good thing. Even in Pittsburgh with our fairly craptastic public transit system, it is realistically possible to now live in the suburbs without owning a car. How that's a good thing for car enthusiasts is this. Now you don't need a daily driver in addition to your "baby" in the garage. Once loan/insurance/maintenance payments are freed up from the daily driver, that is more money that can be put towards your collectible. Also, if your collectible is having one of those weeks where it just doesn't want to run right, you're not stranded at home with no transportation.
For me being a skier, I can take the bus over and grab a Subaru Impreza wagon and spin down to the slopes for an evening of night skiing. You have the choice of any kind of car to suit your needs. It also travels with you when you travel. I can go to DC or Boston or Philly and have a car if I need one on the spot. No more Enterprise rentals.
Flexcar greatly underestimated the demand for such a service here in Pittsburgh. They started here almost reluctantly after the city offered them parking spaces and some long term usage contracts. Now, they are adding cars all over the city... 2-4 a week by my observations.
Neither company uses many domestic vehicles, but the variety is such that I can't get too upset about it.
I hope with this merger the car sharing system continues to grow nationwide.