We've hiked this trail twice - August 2015 and Sept 2016. The August hike was green and filled with wildflowers, and Timberline Lake itself is quite scenic. We passed through a few marshy spots in addition to creek crossings. This September felt like such a different hike -- too late for wildflowers but pretty fall-colored ground cover. The trees are mostly evergreen, so no Aspen yellow but still pretty. Our group rated the trail easy to moderate, depending on fitness level.
Beautiful views from the summit, and multiple 13ers close by you can easily get to from the top. We hiked over to one to get away from the crowds (which were pretty relentless like most easy access 14ers). Hike was relatively easy if you're used to the elevation, as parking is over 12,000 ft. Pretty cool mining ruins to check out on the way up.