Don't be. Book it. Antonio will treat you well.
After looking through posts on this board, sent of queries to the three most recommended names. Antonio at Cactus ended up having the best deal for our group (two cars - Jeep and Minivan - 12 days) and so we went with that. Booked about 2 months in advance.
A couple days before flying down to the Cabo airport, I sent an email to Antonio just double checking we were OK with our reservation. I never heard back from him. That made me a little nervous - but whatever, it wasn't worth sweating over too much.
After clearing customs, we found the Cactus gal outside and she told us that the Cactus shuttle was busy or broken and that we'd just take some taxis to their place - they would reimburse us whatever it cost. There were nine of us plus a lot of luggage (kite board stuff) - but we ended up all fitting in a big cargo van. Bueno.
We got to Cactus and I had my first "uh oh" moment when we pulled in to their lot and there wasn't a single car there. None. Hmmmm.
Long story short - it was Easter weekend and they ran out of cars. To fix the problem, Antonio said that the driver of the cargo van could take us to our intended destination (La Ventana/El Sargento - 2.5 hour drive) and that he would have people drive our cars up to our house the very next day. We ended getting a very significant discount on the cars from Antonio - enough to cover the taxi plus the missed rental day plus extra.
You know what? This actually worked out great. Probably better than if the cars had actually been there. We ended up on the road about 5 p.m. and, frankly, I didn't really relish driving all that way with any hint of having to navigate in the dark on the road between San Antonio and El Sargento. Plus - I was extremely thirsty for some beers and our shuttle driver stopped at a minisuper on the way and we loaded up. The drive ended up being a lot more relaxing (except for the 80 mph turns and passing a police car).
Per his word, some Cactus employees showed up with our vehicles the next day. It was a little tricky since where we were didn't have good cell coverage and was kind of hard to find. When the guys got lost, they couldn't get in touch with us, so they ended up picking a guy up off the side of the road in La Ventana who knew where we were and he got them to us.
At the end of the day - Antonio came through and dealt with a tough situation like a professional. I would not hesitate to use him again.