Took my son for 4 days of fly fishing in the Boulder Mountains the last week of September. It was his first time fishing with a fly rod. We hired three of the four guides in the area for one day each and then spent one day fishing on our own.
I definitely recommend hiring a guide your first time fishing in the Boulders. There are many fishable waters, but if you want to catch the big fish, the guides will put you on them. If you choose not to hire a guide, you will likely spend lots of time exploring and lots less time catching fish.
My experience is mine alone, but my recommendation would be to use the “Quiet Flyfisher” out of Loah, or Steve Stoner of Boulder Mountain Flyfishing. We had amazing days with both of these guides. Steve probably knows the mountain the best, but is very careful about giving you any information on waters you are not fishing with him that day. He has been guiding and exploring the boulder mountain for more than 25 years. Mike James of the Quiet Flyfisher was much more willing to provide some guidance about other waters on the mountain and has also been guiding in the area for more than 10 years. I had a very poor experience with the Fremont River Guides. I feel that they were over-priced and more concerned with the financial side of things than with providing a wonderful fishing experience for my son and I. They have been guiding for the least amount of time in the area and are working to make the Boulder Mountains a world-wide destination, bringing in lots of national TV shows and marketing mostly to out-of-state fisherman. In my opinion this is not in the best interests of the fishery on Boulder mountain.
During our four days we fished several of the rivers in the area, two small streams, and several of the mountain lakes. The fishing was amazing. We caught small brookies in the streams on attractor dry patterns, bigger splake, tigers, and browns nymphing in the rivers, and some fat rainbows, tigers, and splake dragging wooly buggers in the mountain lakes. We didn’t catch any of the monster brookies that the Boulders are famous for, but we did see them. Because my son was brand-new to fly-fishing we concentrated on numbers of fish rather than catching trophies. I think my son caught 80 fish in the four days of fishing, and several were quite sizeable 
. We were there Sept 21-24 and the trees were just starting to turn color. I am sure this recent storm has left some snow at the higher elevations, but the fishing is definitely going to be great through the end of October.
Here are a few pictures of our trip:
