I had a travel system and I really liked it. DS was born in March and it's really cold here until about June so I used it. I didn't have it the first month and I felt really liberated once I got it.
Reasons why I felt I needed it:
It was so cold, I liked that he was bundled in the warm car seat.
DS was a really cranky baby, and IF he slept, we were NOT waking him.
He was heavy and I had a hard time carrying the seat alone.
(11lbs at birth, 22 at 4 months)
I had the slings and used them, but I was a new mom and had a hard time putting them on in the car and again, it was really cold out.
Plus if I was at the mall, I was trying on nursing bras and couldn't wear him at the time.
Before I got the stroller to go with the carseat, I basically put the seat in the cart at the mall. This worked really well for groceries, and not so well for the mall.
Reasons why I might not use one again:
I hated how huge the stroller was that went with the carseat - bohemouth!
I stopped using it once I got my Ergo carrier - and once he really liked a carrier.
I only used it 4 months, before A: DS outgrew the infant seat and B: the stroller that came with it was no good for a smaller child without the carseat, and not comfortable now for a big boy on his own either.
Once summer came, I preferred to carry him without the seat - might have had something to do with how heavy the dang thing was!
To sum it up, I LOVED the stroller system for 4 months that seemed like an eternity at the time, I felt free to go to my LLL meetings and to the mall and to get out of the house with my son in a way that I didn't before I bought the thing.
Hated the stroller afterwards and have had 4 other strollers before finding the RIGHT stroller that I adore. Next time I will buy a baby bucket carseat that fits my new stroller, but if I had a summer baby I might not bother.
Plus, by then I am sure it will be more convenient to carry baby #2 in a sling and push my big boy in the stroller!
BTW - my dream stroller is the City Mini by Baby Jogger! LOVE IT! But it took a couple of crappy strollers before I even knew really what I wanted and needed in a stroller.