I had a baby jogger city mini double, personally I didn't like it. My premies looked so uncomfortable in it. I think its made for toddlers(it's easy to fold and take with you) but not good for daily use. I believe you need an attachment for it to work with any car seat brand. I didn't even bother with it because I ended up selling baby jogger and bought Stroll-Air Duo.
For the single stroller I got Maxi Cosi Forey, you don't need any attachments, just take off the toddler seat and put on a car seat... Made my life easier.

GBB triplets born @ 35 weeks 5 days on 03/24/2011
Triplet A 3 lbs 14oz
Triplet B 3 lbs 13oz
Triplet C 5 lbs 4oz