So, to provide some basic background, I've tried the Shu, the Shiseido, and the Kevyn Aucoin eyelash curlers. While the Surratt is my favourite, the Kevyn Aucoin is a fairly close second. The Shu and the Shiseido really didn't work for me for reasons I'll get into below.
My eye shape: I have very large, wide eyes that skew a bit rounder than the classic almond shape. They're not particularly deep set or protruding. I also have a defined crease. My lashes are super long but annoyingly straight and they're terrible at holding a curl. So my requirements for an eyelash curler are:
a) arc needs to be long enough to fit all my lashes into it;
b) I prefer a less curved shape (ie flatter arc) rather than an extremely rounded curve as the latter tends to pinch the outer edge of my eyelids; and
c) able to deliver a natural curl, not crimp my lashes into an L shape.
The Surratt curler is interesting because it has a very wide opening and a super long arc but the curve of the arc is pretty flat (esp compared to the Kevyn Aucoin). To me, this is perfect because the opening is able to comfortably fit all my lashes in at once and the shallower curve doesn't pinch my outer eyelids (which was my major beef with the Aucoin). As for the Shu and the Shiseido, I found those both extremely uncomfortable and they always pinched my lids. Both also tended to crimp my lashes into an unnatural ninety degree L shape instead of giving a natural gradually sweeping upwards curl. I don't believe that the Shu and the Shiseido are bad curlers, btw, just that they are not suited to anyone with large rounded eyes. They would be good for almond shaped, small to medium sized eyes. The shallow arc of the Shiseido would work really well for monolidded eyes I think. If you have large round eyes like me, then I'd say go for the Surratt unless you have very deep set and/or heavy-lidded eyes in which case the Kevyn Aucoin would probably be the best pick.
Anyway, back to the Surratt. It's a very sturdy well constructed curler that feels more substantial than any of the others. It isn't floppy and loose with a tendency to swing all over the place (if you've tried the Shu, you probably get what I mean). The Surratt also has a firmer rubber pad, versus the more springy ones in the other three. I prefer the firmer pads of the Surratt as they are much more durable and continue to be effective for 6 months+. The flimsy construction of the Shu combined with its springy pad meant that it stopped being effective for me after 2 months. So there's an appreciable difference. And with the Surratt curler, you barely have to apply any pressure to get a lovely, curled lift to your lashes. Felt weird at first since I was so used to clamping down hard but the Surratt is constructed in such a way that you don't have to do that. And the curl holds, people!! Even after lashings of non waterproof mascara.
Oh, the Surratt curler also comes with a spare rubber pad. I don't think that you can buy replacement pads unfortunately but in my experience, most eyelash curlers get looser over time so buying a new one isn't so bad.
Cons: None. Okay, if I wanted to be veeeeery nit picky, I would say that I miss the red strip of the Kevyn Aucoin curler which enabled me to see whether all my lashes were positioned correctly. With the Surratt, the matte gunmetal finish makes it more difficult to see what you're doing and also tends to show eye makeup buildup very clearly (not necessarily a bad thing, but I'd prefer not to know just how badly I need to wash my tools every time I use them!)
To sum up, this is my favourite curler to date and I really notice a difference, both in terms of enhanced effect and comfort level. That said, I don't believe there's any one eyelash curler out there that is going to suit everyone. It really depends on your eye shape and, for most of us, finding the 'one' is gonna require some trial and error.
One tip that helped me determine that I had round eyes rather than almond shaped (which is the default category most of us assume we fall into) is this test thing I read...well, somewhere online. That's about as specific as I can get lol. But yeah, basically look in the mirror and see if the whites of your eyes are visible below your irises (the coloured part). If you can see the whites, then you have round eyes - if you can't, you probably have almond shaped eyes.