We've probably used Hertz seats a couple times a year on average for the last 10 years, all within the US.
One time several years ago I got a filthy seat from a tiny Manhattan location, and they were very apologetic about it. But in the last few years they've all been very clean, reasonably new-looking bare bones Cosco Sceneras (like the $40 Walmart version). I like our own Britax seat better (it feels more solid, has better fabric and hardware, and the kid sits higher), but our kids have never complained about the rental seats and I've never felt that they were unsafe. We actually had a kid in a rental seat when we were in a crash last year, and the seat did its job (thankfully a bruise on my wife's elbow was the only injury in a crash with enough force to destroy our front axle and spin our Chevy Traverse 120 degree).
One thing to know: for liability reasons Hertz will not install the seat for you. The Sceneras aren't hard to install, but make sure to take your time to do it right.