The Shelbys are several years old now and there's a possibility they might be getting retired.
They're probably ~40-60K+ miles by now, and from what I've seen, lots curb rash, peeling stripes, other typical rental car damage.
Hertz also replaces the original high-performance tires with the cheapest off-brand ones they can source.
Just a word of caution you may not get the full advertised bang-for-your-buck.

However, if you just want to hear a V8 sing, and get a little sideways on those cheap tires, it'll do just fine.