I've had these irons since May, and had to take two months off due to a recurring right-thumb injury. Still, I've had three good months of practice and play and share my experience.
I play 4-SW, all with Wishon stepless steel shafts. Prior set was Mizuno JPX 850 Forged, prior to that JPX 800 Pros, prior to that MP-57s... Mizuno stuff. You get the idea.
A summary of my experience is this - I LOVE hitting my 4i. I used to feel a little dread being too close to hit hybrid, but too far to hit something like a 6i. Now, it just doesn't matter. Every iron is my favorite iron. I cannot imagine ever going back to variable length clubs.
The long version:
I fooled around with True-Length-Technology back when that was a thing, because the idea of having irons that "felt the same" was very appealing to me. With TLT versus simply MOI-matching a variable length set, the stance was closer with each club since the length variation was reduced. I went further and did some trigonometry to calculate what lie angle each club had to have in order to create the exact same hand-height at address - an effort to create the exact same stance with every club. This sort of worked, but it required SIGNIFICANT lie bending in some of the clubs, and builders get anxious bending an expensive iron head 5+ degrees.
So, when single length came back, it was a "duh" for me. Makes perfect sense. The only plausible reason not to go SL was the swing speed required to elevate the low-loft irons, and the Sterling irons solved that with their hi-COR faces in the 4-7i. It absolutely helps. You still need some power to elevate the 4/5i, but not near as much as you would without the hi-COR faces.
I can't overstate the value in having every iron play from an identical stance and require an identical swing. My 4i swing is my SW swing is my 6i swing is my 9i swing, etc. The different technology in the low irons takes care of everything else. The confidence this built seemed to free up my swing even more, and I'm hitting EVERY iron better, longer, more consistently than I ever have. Yardage gaps are predictable, consistent, and desirable.
Two highlights from this weekend -
on a 530yd par 5, I'm 220 out for my second shot. Water left, and yucky lies to the right of the elevated green. In the past, since I'm too far out to hit hybrid and I didn't feel very good about my ability to hit my 4i that far very accurately, I'm laying up to a safe distance and playing for birdie. Now? Nah. With a L-R crosswind and hitting from an uphill lie, I mash a 5i that is in the air for what felt like ten seconds and ends up 8 feet from the hole. Tap in birdie.
Later, on a 370yd par 4, there's a nice tailwind. Driver would be too much if it isn't perfect, and 270 is the edge of the fat safe spot. I could reach 270 with my hybrid in this tailwind. A free, pure 4i swing (which is, fortunately, a free pure every-iron swing) and I'm 264 yards down the pipe. Perfect.
The high-loft irons and wedges do go HIGH, probably due to being longer than normal, but this has not bothered me at all. I can hit low shots or knockdown shots as required, but I am happy to let the ball soar even in TX winds. In the past I would never hit a full SW from the fairway - too afraid of catching it thin. Now? You can imagine. It's my 4i swing, and I love hitting my 4i.
I'll touch on the feel of these clubs as well - it's fantastic. The high loft irons are soft, and the sound with a pure hit is a wonderful "thwack" sensation. The low loft irons sound more like a hybrid or even a 3W due to their hi-COR face - it's a more noticeable "tink," but it is at least as pleasing if not moreso. The contact is so soft and satisfying, and the ball is GONE. Being able to hit a ball that far with a 37" club is something else.
Honestly, I think every single golfer would benefit from these irons. If you don't have the swing speed to play the 4i, or the 5i (there is a hybrid 5 available) then you probably can't play your variable length 4i or 5i effectively either. You're simply introducing fewer variables to your iron game. Sure, if you practice enough you can get very good with variable length irons - but even great mathematicians will tell you that equations are easier to solve with fewer variables and golf has enough variables as is. The few "but what about this?" arguments for single length have been accounted for in the Sterling set.
I cannot imagine why I'd ever play another set of irons.