I agree with everything that Maezde said. I just want to add that I got a rock solid install with the seat belt so rock solid installs with seat belts are possible (probably depends on the car).
Yes, this. Almost all of my installs have been with a seat belt, and I've been pleased with their solidity. Although, I did put it in with the SL once for fun, and I was like "oooh..." because it was so easy, and solid, lol.
The biggest cons are the amount of RFing room it *can* take, and the price. If you (a general you) budgeted for a Britax, though, then you can easily find a RN on sale in that price range.
As someone who cares for tall/heavy kids, and is dedicated to RFing practically as long as possible, it's a no-brainer. I actually love the fact that it's heavy; it's solid and I feel like my kidlets are very safe in this seat.
I've never used it with a little, though... it seems silly to put the lighter and smaller kids in the RN when I have preschool-aged kids who can use it. Although I did put my 22lb 2yo in it last week, and she fit well, despite the fact that I was amused at the six miles of shell above her head.
One of the things I want to do in the next week or so is try my 63lb, 48" 6yo in the seat, just to see how he fits. He's AT both the height and weight limits of the Nautilus, so I'm interested to see what he thinks about the RN. (He typically rides boostered, so it's just out of curiousity.) I would expect you could use it from newborn to booster age, including ERF, without a problem.