Not having a car has been going very, very well for us. It seems normal to walk, ride the bikes, or take the T to where we need to go. Every once in a while, about 2 - 3 times/month we've needed a car and we fill that need with either Zipcar or a regular rental.
There are some annoying things about rentals though, that take some getting used to. For Zipcar, the annoying thing is that your rental is limited to 180 miles total for your rental period. If you rent a car for a day, it is really, really easy to go past 180 miles. If you go over you have to pay by the mile - like $.45 per.
For the regular car rental you are subject to the laws of supply and demand. If you want to rent a car on a holiday weekend (say.... Thanksgiving), you are subject to much higher rates. The best rate we could find for the upcoming holiday for a 4-day rental was $343. Normally that would cost us about $160.
Oh well. Now that we know, we can start planning a little better.