I did this for a while after I moved to the bay area. Turns out, it adds up quickly. A weekend of climbing or backpacking or skiing comes in around $300. On top of that, it's less practical. You have to go get the car, higher mileage trips cost more, you can't rent for longer than a few days, etc. If you're going out of town every weekend, and possibly driving during the week too, car sharing services aren't quite as great as they seem.