It's interesting what effect a small change in reach can have on the handling of a bike. I've been riding the Crumpton around since Thursday, so a little under 250 miles, with the new Di2 group. Since the reach to the levers is a good bit longer than SRAM eTap, I opted to go with a 120mm Thomson X4 -10 stem instead of the -17 X2 I previously had. My reach to the hoods is roughly the same as before but my reach in the drops is now around 13mm less. The bike feels a lot better on the descents. Whereas before it felt like it lacked agility and nimbleness, it now feels a lot more responsive. I took it down the same descents that I did with Tonytourist a couple weeks back (Rock Store and Fernwood in the Santa Monica Mountains) and had a blast. The 9100 brakes were also impressive. A bit of a weight penalty (or a huge weight penalty) but on steep technical descents like Fernwood, I feel like they make a big difference. I also managed to shave off 18s on a 5 minute descent over the last time we did it.
As far as 9150 itself, I'm really pleased with it. I like the feel of the shifters better than eTap as well as the levers. Being able to manipulate the RD with one hand is nice especially when eating or riding on the tops. Front shifting is also a lot better. My only gripe is that the RD placement interferes with removing the wheel. I'm kinda scratching my head on that one. I was barely able to get my Enve with 27mm tires.
Here's the current build list:
_________________StravaCurrent Stable. The Snob Machine
| The CrumptonThe Ex's.
LS Siena: 6.21kg | Parlee Z5 SLi: 5.9kg | LS Xicon: 5.76kg | C59: 5.7kg | Cervelo R5ca: 5.09kg | Fuji Altamira SE - 6.2kg | Scott Foil - 6.2kg | Evo - 5.18kg | LS Classic - 6.7kg