Just finished this loop. It's been described before, but here are some quick thoughts:
I used the Aspen/West Maroon Lake trailhead. I had no trouble driving straight to the trailhead at 4:30 on a Friday afternoon. I didn't inquire about the official policy of who gets to drive and when vs. who has to use the shuttle--I just showed up, told the ranger at the gate I was backpacking, and he waved me through without comment.
I did the basic trail loop clockwise with no bells or whistles. I camped first night just below treeline north of (i.e., before) Maroon Pass, and I camped the second night just below treeline west of (again, before) Buckskin Pass. The valley leading up to Maroon Pass was incredibly crowded with other campers--I was grateful for the flexibility in campsite selection that comes with a tarp/bivy combination. The area around Snowmass Lake was similarly crowded. The small basin below Buckskin pass was gorgeous and completely empty on a Saturday night. Perhaps because of the times at which I hiked, I didn't run into too many people on the trail except for on Sunday morning, when I hit hordes of incoming once I got to the valley floor.
I spent 41 hours on this route, but it could easily be faster or slower. For me, my pace was dictated by work (couldn't make the trailhead until 5 p.m. on Friday and needed to be out early on Sunday--I left at 9:30 a.m.) and concern that I shouldn't commit to a schedule that would require crossing any passes in the afternoons, for fear of thunderstorms. On the two days I was out, however, the weather wouldn't have been a problem. I got a light sprinkle both afternoons, and substantial rain on Friday night, but I could have come out Saturday night (doing Buckskin pass after 6 p.m.) without any problem. But I didn't have anywhere better to be than that basin. On Saturday I had plenty of time to take some breaks and shoot pictures--in trying to learn to use a borrowed SLR, I shot over 200 photos on the trip.
If I had more time (which I originally thought would have been the case for this trip), my first choice would be to combine the east and west approaches to Snowmass Mountain, substituting that summit for Trail Rider Pass. For someone with my level of ability, this would be possible on a two-night trip with a full day at the beginning (rather than my short days on each end): day one would be hiking clockwise to Lake Geneva, to queue up a west-side approach of the summit; day two would be the summit and, if I was up to it, a hike to the basin before Buckskin Pass; day three would be a hike out, with a detour over Willow Pass if I'd made it all the way to Buckskin the night before. Of course, I ran into one couple that explained that they were doing the whole trail loop as a day hike, and I saw a bunch of trail runners that didn't explain where they were going, so it can obviously be done faster. There a ton of other ways to stretch this out as well.
The trail conditions were great: no logs across the trail, little brush, and mostly dirt footing that was kept firm and non-slippery by the afternoon rains. I did the hike in road running shoes with poor grip without any problems (having destroyed my trail runners the weekend before).
Mosquitoes were not a problem--I think I noticed two on the whole trip. I slept with my bivy unzipped and had one spider on my face in the night but no flying annoyances.
I used a bear bag rather than a can, and finding appropriate branches to hang it from was a minor challenge--took me ten to fifteen minutes of looking around each night to find a location I thought was acceptable.
Water was available all over the place. The only time I was worried I didn't need to be: on the hike from Snowmass Creek up to the basin before Buckskin. Based on the topo contours I figured there would be water in the basin, but I got skittish about it spent a solid 15 minutes or so trying to collect water from little muddy trickles in fear of a dry camp, only to find abundant water at my campsite.