Has anyone used the Joovy Caboose (I'm looking at the Ultralight version I think) with a newborn and a 3 year old?

My son is probably starting to outgrow the stroller stage, but he thinks running away from me is hilarious, so I need something that I can put him in when he runs. He also occasionally likes the break the stroller offers. We bought a Britax B-Ready for our son with plans to use the attachments to create a double stroller in the future. Unfortunately, it looks like Britax has discontinued the accessories for the 2012 model and I can't find the second seat anywhere online. So, I have been looking at sit and stand strollers, but I think we are pretty limited in options by our infant car seat (Keyfit 30). I like the look/specs of the Joovy, but I'm a little concerned by the space the car seat takes up. If you used a car seat with the Caboose, how is the space in the back seat? Do you feel like it works well for walks/longer outings?