I could go on and on about carseats, so I'll try to keep it short :)
For my 1st I used the carrier a lot. I brought it everywhere. I did not, however, use a baby bjorn, or sling or anything like that. If I had been more comfortable with that, I don't think I'd have used the carrier as much.
The seat I had didn't last long, so I moved to a convertible seat (the britax roundabout) when she was about 6 months or so.
I borrowed a carrier for my 2nd daughter to use for about 2 months, then bought another convertible seat.
For my 3rd, I bought a new carrier seat. It is one that goes up to 30 lbs. She is 8 months old and about 20 lbs and I still use it. But, I don't actually carry it much. I put it in the snap and go or in the grocery cart or bring it right into the house or what have you. It is pretty heavy, but you can leave it in the car if you want anyway.
To put it into perspective, though, you are going to have to buy a forward facing seat at some point. Many of them, like the roundabout, can be rear facing as well.