We'll be staying at Quepos next month. Was thinking of taking the zip line tour. How safe are these tours?

Just go with a reputable company and they are very safe...check out this one: http://www.midworldcostarica.com/
Enjoy!
I have been to 3 different ones in that area and they are all very safe. If you want alittle more than zip lines try ADR Adventure Park. I just did that in Feb and it was awesome. Give it a try and have fun.
Just did it with Quepo Canyoning and they were great, super friendly and very safe. Their gear was in good condition.
We just came back form Quepos last week. I found that you get much better deals once you are there. Surprisingly (or not) our hotel's front desk had the best deals and we were quite happy. We did a tour via Tico Tours and they were really, really good.
If you get a chance, hire one of their guides for the Manuel Antonio National Park hike. Well worth the money and much cheaper than the ones we saw online. They also have canopy tours.