Just got my 15x12 for my Sovereign GT. Haven't used it yet but from most threads I've read on the net people prefer it over the WOT. Deeper, more sensitive, pinpoints better, more stabile allowing higher sensitivity settings, and seperates better. Many remark at just how sensitive it is to tiny objects like earing backings and that it seperates really good, allowing them to hunt in much more trashy areas than they first expected. Also read of many who remarked that they were surprised to not only find deeper coins at their worked out spots, but also to find shallow coins in trash or standing on end that somehow this coil unmasked while others didn't. That reminds me of the old reports on the WOT but they seem to be even better for the SEF.
Here's a few pictures of it compared to my 10" (not 10.5") GT coil. The one with the 10" on top of it has both coil's bolt holes lined up for the most part. Shows a little more weight towards the tail end, which is good because I think that would put less stress on your arm. Coil looks of very good quality and many say it's the deepest coil on the market while still retaining sensitivity at those depths to coin/ring sized targets.
Also, here's a coil weight chart I put together. A "W" means I weighed the coil. A "*" means somebody else did this for me. Most of these are Sovereign coils but a few FBS ones are listed as well and noted. As you can see, the 15x12 is lighter than the WOT. In fact, if you were to use it without it's coil cover it's .6 ounces lighter than the stock GT 10" coil with coil cover. For that reason I amy leave my cover off and spray the bottom with spray on bedliner like a lot of beach hunters do and get it's weight to about that of my stock coil. Not bad when you think about it for this size coil.
I almost bought the 12x10 and am glad I didn't because the 15x12 is smaller than I had pictured in my mind. If I would have bought the 12x10 I would have been let down by it's size in comparison to my 10". I think I must have been looking at photos of the 18x15 on the net and that threw off my size perspective. Don't get me wrong, it's a big coil, but not nearly as big in person to me. I also thought it would feel much heavier than it does in my hand, though I haven't hunted with it yet. I am making a lightweight custom shaft for my GT and have already saved more than the weight of this coil over my stock coil, so my machine is already going to be lighter than a stock GT using the stock 10" coil. I have plans to make my GT even lighter with a few other mods too.
By the way, crunching the numbers I think the extra coil cable length on a GT needed to hip mount the box is roughly about 3 to 4 ounces. Since I'm building my custom GT light to avoid having to hipmount on those longer days I might just wack this coil cable down in length. That alone will get it to about the 10" coil's weight, and even less if I opt not to use the coil cover.
I had my concerns on all these larger coils and spent the last few months digging into the dark corners of the web looking for any and all information on all the larger aftermarket coils, from the S-12 to the round Excelerators, WOT, and of course the SEFs. I wanted max depth on coin/ring sized starts while not losing any sensitivity to them at max depth. If you read the "official" field test of this coil you might think the 12x10 is deeper on small coins. Well, it is by half an inch on a cut quarter penny, which is probably half the size of a half dime. For a while that turned me off until I saw a picture of one of those "coins". Any smaller coil will get deeper on something that tiny. From what I've read this coil will get deeper than any other on any penny/dime sized coins/rings or larger, and for me that was the final straw.
I will tell everyone right now, it is a heavy coil mounted up to the E-Trac, I never get detecting fatigue (My back gives out way before my arm ever does) but this one is going to be a workout. It is at least twice as heavy as the Pro Coil. Really looking forward to anything else you may have to say about it Ray 
I hate when you type up a detailed report or letter only to lose it. 
And for that matter I like to hear what they don't like as well in detail on coils or detectors.