The bike is well proportioned, and the XL size was a comfortable fit on steeper, tighter tracks, although it felt slightly short for me in some places. The 461mm reach isn't huge, but being a 29er it still sizes up more spaciously than smaller wheeled downhill bikes due to the higher stack height. The 453mm long chainstay had a good balance with the 461mm reach, and as long as the trail had a decent pitch to it I felt centered and stable in the middle of the bike.
When doing back to back runs with the other 29er DH bikes I've been testing on the same trail, the Session felt less planted at higher speeds, especially on straight lines when you just want to let go of the brakes, hold on, and charge through. In these sections it was also harder to keep my feet in place on flat pedals. The reach may have been a factor here, but I believe that being the lightest 29" downhill bike I have tested this year also contributed to feeling less planted.
As with any bike that has a good balance front and rear, and equally adjusted suspension, the Trek got airborne easily with no surprises. Coming in to really heavy landings the bike took the big hits without question. If you went too far though, the Session 9.9 did have a fairly harsh bottom out—the bike now comes with the 2019 version of the Float X2 shock, which has a much bigger bottom out bumper and should help avoid that.
The combination of the bike's light weight, suspension design, and massive stiffness makes the Session pedal and accelerate very well. It also rails fast, smooth berms exceptionally well, and it's easy to see why some riders have had success on the World Cup circuit with this bike under them.
That said, heading into the rough stuff I found the Session noticeably, well, harsh. Tracking across off-camber sections and carving flat corners didn't inspire confidence - the rear wheel had a tendency to lose traction when the bike was leaned over. Despite the 'Active Braking Pivot' design keeping the rear triangle active under braking, I found the braking traction lacking, with the bike skittering into rough corners when I was most looking for grip.
The harshness I experienced may have been affected by settling on slightly less pressure than recommended by Trek, with the theory being that I could have been running deeper in the travel than necessary. That said, for the terrain I tested on I generally prefer a softer setup and the Session was set up similarly to the other bikes I tested. Alternatively, a coil shock could be the ticket here, which could improve the sensitivity and tracking of the back end of the bike.
Similar to what I experienced with the Norco Aurum
, there was also some vibration from the fork in the lower setting when riding mellow bike park trails with small stutter bumps. I'd put this down to the slack head angle and the 29" fork being long and riding high in the travel on flat terrain. I generally set the bike to tackle steeper terrain where this issue didn't appear, but flipping the Mino-Link to the high position helped, and if I was riding less steep trails more often I would be lower the front end of the bike through the crowns and raise the stem or handlebar to keep a similar riding position. I tried this in the Whistler Bike Park, where the trails aren't quite as steep, and it did end the vibration issue, but then I was in need of a higher rise bar or direct mount stem spacers to maintain my preferred bar height.