We have 2 car seats already which we registered for because when in the store, they showed us "how easy" they are to take out and move to another car... But, more and more, I'm thinking we may need 2 more for hubby for when the daycare drop offs begin, if we're going to be taking turns...
How many do you have and does it work logistically?  And what do you recommend?  We currently have 2 Evenflo Symphony 65 - Liberty. 
Re: Recommend your convertible car seat & do you have 2 or 4?
Right now we have two convertible seats in my husband's car and we're still using the infant seats in my car. But they're outgrowing the infant seats and we'll be getting two more convertible seats (for my car) really soon.
My husband does the drop-offs and I do the pick-ups. So we each need a set of car seats. It's not optional for us.
we have britax in my car for all 3... in DH's car we got the Cosco scerena (sp?) which are good b/c they are cheap, have good safety ratings, and are pretty thin so fit 3 across in his Ford Escape.
I would not want to have to remember to take the seat in/out of my car... i think it's worth the money to get 2 more, even if you hardly ever use them (like we hardly ever do in DH's car).
we have graco comfortsports for my mom and sister's car - each have 1... from when we only had Griffin. Hopefully they won't ever need to drive the twins anywhere- but at least they have one seat avail if needed in an emergency for one of the babies.
After 2 rounds of IVF & 2 rounds of FET, we were blessed with identical twin girls!
***Twin fraternal girls born at 35w6d in 12/2008***