Is it safe? Yes. Petty thieves? Yes. Take nothing of value. Whatever is of value needs to be on you at all times.
They know the deal with rental cars so don't ever think your stuff is safe because it's locked in the car, it is not. The best way to avoid your vehicle getting ransacked is to leave the windows down.
Last time I was there I watched my flip flops get ripped off when I was surfing. Saw everything, he looked right at me and shrugged knowing I wouldn't get back in before he was gone. They were cheapo's, I expected it to happen. I wear a size 13, Ticos do not, it doesn't matter though.
Other than the petty theft, it's a wonderful place!