Like its predecessor, the Fluid Flex ST is another tester favorite. What stands out about this shoe is its comfort and light weight. The feel of the shoe on the foot is nothing short of dreamy in all respects. And it feels just as good at recovery effort as it does pushing the pace during a tempo run. This is because the midsole material, despite its soft feel, is actually quite responsive and energetic. This is a hard combination to get right but the Fluid Flex manages to pull it off without the stiffness usually present in racier shoes. The Fluid Flex does have a touch of squishiness initially, but this quickly goes away with break in. 

The Fluid Flex is not quite at home on technical terrain. The outsole struggles to grip on loose surfaces, and the forefoot is just a bit too flexible and loose fitting for hard, aggressive running. The exposed cushioning material on the middle third of the outsole also reduces protection. 

The Fluid Flex will appeal to runners looking for a lightweight, cushioned platform to use as a daily trainer or for up tempo running on gentle to moderately technical terrain. Runners with narrower feet may also find the Fluid Flex to be an excellent long run shoe. The Fluid Flex is probably not the best choice for highly technical trail and off-course mountain running. 

Comfort
Comfort is the Fluid Flex’s strongest feature. It feels great out of the box and is comfortable enough to wear all day. The upper materials conform to the foot and breath very well without being stiff, and the midsole cushioning is right in the sweet spot between too mushy and too stiff. 

Speed & Agility
The Fluid Flex performed amazingly well when cruising at medium to faster speeds on less technical terrain. Speed could be enhanced with a larger heel-to-toe drop, but otherwise the responsive cushioning and lighter weight make for a shoe that is very easy to turnover. Precision foot placement is no problem due in part to the narrower profile. 

Security of Fit
The fit of the Fluid Flex in the heel and midfoot area is excellent. Unfortunately the fit in the forefoot was not quite as good and proved hard to dial in, even with the laces tightened down. The main issue seems to be that the structure of the shoe simply does not provide enough lock down. The softness of the cushioning also plays a role here, although this aspect of the fit did abate somewhat as the shoe became more broken in. 

Stability
The Fluid Flex struggles somewhat on technical terrain due to the rotational flexibility and fit issues. These characteristics made for tentative running on challenging terrain. The handling did improve with time as the cushioning compressed down a bit and developed more rigidity, but it was still mediocre at best.