By way of background: I have been kayaking once or twice a week for about 18 months. Entirely self-taught. Switched to greenland paddle a few months ago. Briefly paddled a Delta 12.10, then switched to an NDK Explorer. Generally paddle the great lakes and inland reservoir.
I've been paddling an Illusion since June, but didn't feel completely ready to give a review until I just paddled in Lake Erie winds documented in the 32-37 knot range.
This is an absolutely wonderful kayak! The comfort of the boat is tremendous. The deck will be the right height, since you order it as such. The seat is simply perfect, both in comfort and support. The foam of the seat gives traction to your bottom, aiding in control of the boat. I strongly urge you to get the custom bulkhead instead of the footpegs if you plan to keep this a long time (and you will, once you own one); the angle for your resting feet and the freedom to put your feet anywhere across the bulkhead is simply far superior to foot pegs (althought the ones used by Sterling are great). I also love the lack of a backrest. The seat provides appropriate low-lumbar support, allows easy lay-backs, and does not hinder re-entry-and-roll. Bottom line: this thing feels like it is simply an extension of your body.
As far as performance, again, it is amazing. Compared to my Explorer, I find it is easier to place on edge (less initial stability), yet more comfortable keeping it there (better secondary). It turns quite easily. I also find it far easier to paddle backward vs. the Explorer.
The Explorer is well-known as a great-rolling kayak, and the Illusion is even better. Rolls are simply effortless. And it is very comfortable to lay back onto the back deck.
It is very well-behaved and forgiving in surf. I had never paddled breaking surf before the Illusion. Took it out in 3-foot surf in North Carolina, and was completely comfortable surfing and side-surfing. Surfed a couple hours without capsize; the next day did the same using a greenland paddle, and I had only started using a greenland the week prior. The fact that a complete surf-virgin could do this speaks volumes to its surf capability. Since then, I have also found it is easy to prevent broaching with a little lean-and-stern rudder.
The other day while out paddling in 32-37 knot winds on Lake Erie, it was very easy to control the boat (using a greenland paddle). I had no problems turning into the wind or downwind (but plenty of problems paddling into that wind!) or paddling with a crosswind.
As far as tracking: the only time I notice weathercocking is in moderate-to-strong winds on a local 1x1.5 mile reservoir, where there is not significant waves despite the wind. While on the great lakes or ocean, seems to track fine regardless of wind strength or direction. The skeg is an improvement over the Explorer. Far less likely to jam, but it still does sometimes with coarse sand/tiny gravel. I like that you can still turn easily while the skeg is down.
This is a relatively low-volume boat. Despite gaining a bit of storage space by having the front bulkhead moved back considerably (I have short legs), to pack for a weeklong trip you really have to pack like a backpacker. One thing that does help is that the skeg is tucked flush against one of the bulkheads. If having more storage volume is really important to you, you may want to consider Sterling's Grand Illusion.
This is clearly an amazing boat for an intermediate to expert kayaker. But I would not hesitate to get one if you are a novice... IF you are are a quick-learner and anticipate putting in the work to become an advanced paddler. In any case, it is should be easier than an Explorer for such a person. My kids ages 9-12 paddled it this summer, and they are complete novices.
Overall, this is a wonderfully responsive kayak. It accomplishes what I feel is most important in a kayak: it is very transparent. By that I mean that it paddles in a way that allows you to almost forget you are paddling a boat. Everything is so natural, comfortable, smooth, and effortless... it's like the boat is just part of your body and responds to your mind the way your arms and legs do. Just like I don't want to notice a window I'm looking through, I don't want to "notice" the kayak I'm paddling. The Illusion can do that for you.
As far as negatives: 1) the hatches, while implosion proof, do leak. Despite Sterling putting on a "back-up" seal, if I go out and do a couple dozen rolls, the rear compartment will take on approximately 12 ounces of water. In comparison, every hatch of my Explorer remains 100% bone-dry (although it does have a smaller hatch). It just seems to me that if I can buy a $10 watch waterproof to 100m, then a $4500 kayak should have waterproof hatches. 2) Despite having the infused carbon fiber layup, my kayak weighs 44 pounds, not the advertised <40. Not a big deal; hopefully the extra weight translates into a somewhat stronger hull. Still far lighter than my Explorer. Definitely get an infused hull...it is a far superior construction technique (I have not seen any other kayak maker offering this, but the technology is used on some quality boats).
I was one of the first mail-order purchasers (I bought it sight-unseen). The shipper, unfortunately, damaged the kayak. Sterling was very responsive: he began building the replacement before even sorting things out regarding the shipping insurance; he was willing to eat the loss if need be. Since then, he came up with a better shipping crate which should make such occurrences rare. I would not hesitate to have one shipped.
So, I searched long and hard trying to find the "perfect" kayak. I think the Illusion is the closest thing I can find to that (although we'll have to see about Sterling's new "Reflection" kayak...). Best construction technique/materials. Perfect fit. Absolutely transparent handling. I love it.
(Off topic: for anyone who has yet to explore using a greenland paddle, I encourage you to do so. It is similar to the Illusion in that it is very transparent...just a very natural tool. Comfortable to paddle, simply unfazed by any wind strength/direction, effortless to brace from any paddle position. Great partner to a great kayak. I use a carbon fiber paddle from Superior Kayaks...see my review there).