I'm a bit of a researcher, but I wouldn't say I'm a safety fanatic. From what I've read about car seats expiration, it's NOT the straps that are the concerns, it's the base, the plastic (older plastic ages, warps, and/or hairline fractures can develop in both the base and the seat itself), and that safety standards change and improve over time. The expriation dates vary from one model and manufacturer to another, but most average 6-8 years, with the maximum I've found being 10 years. Also, if you're inheriting a used seat from someone else, be sure to ask (and find out if they've been the only user, or if they got the seat from someone else too) if it's EVER been in an accident- because most experts agree that after any accident to replace the car seat as well. Now I'm all for using things again and not having to buy new at the whim of some frenzy, but it's also good to know why things are reccommended to be replaced.
As for what's so great about Britax? (I'd also throw in here the Chicco infant seat too because the things I love about the Britax also apply to the Chiccco). I have first-hand experience w/ the Graco Nautilus, the Chicco Key Fit, and the Britax Marathon (I own all three). I've also borrowed (when traveling) the Graco up-to-30lbs seat.
1) Installation. While all of them have latch, the Britax and Chicco install SO much faster, and I'm more confident in the security of the installation. This goes for latch installation (my minivan), and old-fashioned installation (my dh's car AND the rear of my van).
2) straps don't tangle. It was SOOO frustrating on the graco infant seat we borrowed (as I was used to the Chicco), when I had to untangle the straps (they kept twisting) at just about every use. My son's graco nautilus also tangles- as in the straps twist within the plastic buckle thing- quite frequently.
3) height adjustment on the straps- it's faster and easier with the Britax and Chicco.
4) the tension/release is smoother (the strap you pull to secure the straps once you've got the child buckled in)- it's easier to pull and get a smooth motion, tight fit in the chicco and britax than it is with the graco. The release lever/button thingy is also easier to find and release. While they both function the same, I find myself having to pull harder on my son's graco, and the release was difficult to find and use in the graco infant seat (though I do admit that could be because I wasn't completely familiar with it and only using it for a short time).
I'll also add as a disclaimer that I am NOT a child seat safety technican of any kind.
I'm also not one of those people who are all about the name brand.
but also keep in mind- as a pp also pointed out- all car seats have to pass the same minimum saftey standards to be sold in the US.
I'm just stating my experiences because someone asked what's so special about them. They are nice- and I'd buy them again in a heartbeat because we use our carseats almost every single day and do a lot of travel to visit the grandparents- both of which live states away. The features I stated above are worth enough to me to invest in the nicer models. BUT, if you can't afford a britax, you can't afford it. There's no reason to go into debt or break your budget just to have THAT car seat. The other models do work just fine to keep your child safe, and that is the bottom line, isn't it?