It's fairly easy to install without the base. Do you have the Keyfit 22 or Keyfit 30? If you have the 22, you will need to pull the cover off the sides to make the belt path accessable. Other than that, all you do is thread the seatbelt through the slots, tighten, and lock the seatbelt. Make sure it's at an appropriate angle and you're good to go. It usually goes in pretty quickly. :thumbsup:
Now, if you're going to be using it baseless quite often (and in the same vehicle every time) you may want to think about getting a spare base and leaving that installed. The more often that you have to replicate an install, the more likely you are to make a mistake when installing it. You just have to be sure that you use the seat correctly every single time you install it. You also need to think about teaching other people who may transport your child how to use the seat correctly. Like I said, if it's going to be in the same vehicle often, it may be worth it for the extra base. The ultimate question though, is, is it worth it to fork out an extra $X amount of money when you can use it without the base? That's up to you.