I work for a non profit solar installer, and our region uses Enphase equipment almost exclusively for our residential projects for a whole bunch of reasons, some of which may not translate perfectly into the for profit world... so your mileage may vary.
I really like the Q Cable in the right scenario. In general we do lots of small systems (most of our jobs are 1 branch). We have mostly been using it where we have 2 arrays on the same branch that aren't located in the same roof plane, and eliminating the Junction Box on the array located farthest away from the roof penetration. I like the fact we are almost center feeding the second array, I like the fact I get to eliminate a junction box, wire nuts, crimps, and pulling a second mod during inspections in AHJ's that want to look in J Boxes, I also like the fact that my crews get a chance to practice with MC4 crimps which they don't see every day. In general it does seem slightly faster to install field wirables vs a J box in my experience, but it's not huge.
It also makes installing systems that are just slightly over 1 branch a little less irritating... I'm building a single roof plane 17 mod job tomorrow. With M250's we may have seriously considered not installing a MaxBranchLength+1 design and just dropping a module because the juice for one extra mod may not be worth the squeeze of upsizing conduit, pulling the extra conductors and barely fitting 6 wire nuts into one SnapNRack Junction box. With field wirables I can take this same 17 mod design and break it up into a branch of 8 and a branch of 9 and land both into the same junction box really easily and not burn extra drops. In fact, with IQ and Field Wirables the only reason I still carry 3/4" EMT hardware is for service on existing systems, training, and the real off chance that we need to install a 3 branch job.
There are a couple other small advantages we've found which may not be applicable in the For Profit world:
We work with a large volunteer base and job training organizations, because we don't have to land EMT into a second J box it makes our conduit runs just a little easier for our crews, who may be teaching folks who have never been on a roof before how to bend pipe in a little less precise application. Also in general, the fewer wire nuts and bare copper crimps we can run is generally a better situation with less opportunity for error.
Our experience may not translate perfectly into the For Profit world, those are just my observations! I wont use them every job, but I'll keep them on my van in case I find a good use for them.