We have the Sit 'n Stroll for our ds (just turned 1). I got it from a friend-of-a-friend for only $60 (although it is used it is in great condition). We have used it for 2 WDW trips and another trip to visit family. Overall, I do like it but not as much as I had hoped and certainly not to the point that I would pay full price for it.
Firstly, as a carseat, it is difficult to use. The harness is extremely difficult to adjust and the seatbelt guides on the seat don't seem to be in quite the right place. The seatbelt seems to span right across my son's midsection and he seems to feel constrained in it unlike his regular carseat. We used it on 2 trips as our carseat once we reached our destination but decided on this last trip to bring and check our regular carseat as we just didn't like the way the sit 'n stroll functions as a carseat in a car (on the airplane it seemed fine).
Secondly, ideally the sit 'n stroll should fit down the airplane aisle so that you can push the baby to your aisle and then install it as the carseat. Unfortunately, out of the 10 flights we have been on with it only 1 time did this work (and only BARELY then). Therefore, you have to carry the carseat over the top of the seats to your aisle which is a major pain (especially since you hardly ever get pre-boarding with kids anymore). This was a big disappointment to me as I had anticipated how much easier it would make our boarding process.
Thirdly, as a stroller, I would say it only satisfies the minimum requirements - it holds the child securely and rolls. Beyond that it doesn't do much. Obviously there is no recline and no canopy (you can buy one but it doesn't look too great) and essentially no storage. It is great for pushing around the airport and maybe for a quick errand but I would not want to rely on it as my only stroller.
So, obviously I still like the sit 'n stroll enough to continue using it. It is a vast improvement over toting a carseat and pushing a stroller through the airport but it does not accomplish as much as I had hoped. I do, however, have a friend who has one and thinks it is the greatest thing ever. Her dd is 3 though so she doesn't care that much about rolling it down the aisle etc. If you come across a good deal on one, I would say it is nice to have but I would not pay retail for it.