Good overall experience with a 1-week car rental....with some caveats. Bottom line - we'll use Sixt again before any other rental service in Puerto Vallarta (or worldwide).
The good: car was in great condition and ready at pickup time. We made the reservation the evening before and the online transaction was seamless. No line at the pickup counter and they upgraded us for free (!)
Not so good: through no fault of Sixt, the transit authority in the state of Jalisco, Mexico *requires* at least 5 mil. MXN of third party collision coverage - this corresponds with the mid-tier insurance option on the Sixt website. Frustratingly, the website still allowed us to book without add'l insurance. Just to be sure on the final price, after the online booking we called and asked (in fluent Spanish) if the price listed on our reservation with basic insurance was exactly what we'd pay at pickup. The receptionist confirmed, and we re-asked the question in a different way to be extra sure. She said "the price on your reservation is what you'll pay at pickup". Then, at pickup, they explained the Jalisco insurance situation, and we had to pay for the upgraded insurance. Seeing we were annoyed by this, they upgraded us to a midsize for free (!), which was nice of them. Clearly they deal with this situation frequently and even had a QR code available to scan and read about the car rental laws in Jalisco.
Secondly, the Sixt website clearly states that pets are allowed in all their rental vehicles, with no comments on extra cleaning fees. Although our small dog has no smell, she does leave fur, and we wanted to be respectful customers so we took the car in for a professional cleaning/detailing prior to the return. To our chagrin, the reviewer found "4 small hairs under the cushion", so they sent the manager out to inspect. She told us cars are only for humans. After we pointed out the pro-pet policy on the Sixt website, they said any pets should always be in a carrier, that the Puerto Vallarta location doesn't accept pets at all, and that we were told this upon pickup (not true). After arguing a bit they didn't charge us an extra cleaning fee, but it added an extra 20 minutes to our return.
All that said, the staff were polite and professional even during this tension, and we weren't charged any unexpected fees, so we'd rent with Sixt again (the other mainstream car rental services in Puerto Vallarta are terrible by any metric - as reflected in their online reviews). Sixt really is the best in the industry.