I had emailed Gus late the week before we left, and unfortunately I didn't get his information in time, so we booked the Los Escobas Waterfall tour through the ship for $63 each. The tour was run by Grey Line Guatemala. Three busses took passengers (40 people each bus) through the town and around some very windy narrow dirt roads to the waterfalls. Our guide was a native of Guatemala and spoke very good English. We were able to swim in several of the pools below the waterfalls, which was very fun. There were many other tours going on at the same time so there were A LOT of people there. After the tour, we bought some chicken and rice from an older woman who seemed to be selling the lunch mostly to the tour guides (we paid $5, although I'm sure the tour guides paid much less). There was also another woman selling a very strange desert consisting of a layer of shaved ice, a layer of stewed tropical fruits, grape syrup, and dulce de leche topping (like a thick, caramelly, sweetened condesned milk) for $1. It was strangely tasty.
It seemed as though many other people simply hired a taxi to take them to the waterfalls and get a tour (I heard they were charged about $30 each). I also saw people from Gus' company giving tours as well. Most of the other guides did not seem to speak as much English as our guide did.
My impression was that the town and surrounding area was very safe (no, the soldiers with the machine guns are not going to shoot at you...), and was actually very charming. The people are VERY friendly and excited to show off their country to you; everyone in the town waved at us at the bus went by. One of the security guards at the shopping area spoke to my wife (in Spanish) asking her how she liked the town, hoping that she didn't think it was too dirty (it was really very clean), and sincerely hoped that she enjoyed her time there. I wouldn't go wandering into town on your own if you don't speak any Spanish though.
Cat25, Here is a blog I came across that hopefully will help. I was interested in the same thing since I will be there in April. Hope this helps!!
In the future, I will definately "Go With Gus" on one of his tours. There was a stand set up in the shopping area where you can set up a tour the same day you get into town. Just make sure to get off the boat early to set it up. It seemed his Rio Dulce tour was running about $75 pp (including lunch), but I've heard great reviews.