My initial impressions and comparison to the BK15 (the only other tweeter I own):
Weight/Balance: slightly heavier than the 15. The balance point seems to be right where my index finger resides on the Harpoon, while the 15's balance point appears to be slightly further back on the handle.
Handle: It's a "tweener". I have small hands, so I've never found the tweeter handles uncomfortable. Although, even I'll admit they could probably benefit from a little more girth in the palm swell.
Blade geometry: It's effectively a high saber grind. One advantage of the wider blade I can see is that the width allows for a reasonably acute primary grind angle and thickness behind the edge, so the high saber shouldn't sacrifice too much push-slicing performance. It would be interesting to compare the primary grind angles of the all the "tweeners", if anybody might have that info.
Without a ricasso, I find no need for a choil. With the BK15, if I want to apply pressure with my thumb directly above where the blade contacts the work, I have to use the choil and place my thumb on the swedge. With the 18, I dont need to move my index finger past the integral guard, and the full spine thickness up to the back of the harpoon is much more comfortable for my thumb.
I find the back of the harpoon to be a nice indexing point for either my thumb when whittling, or my index finger when doing fine slicing and food prep.
The edge appear to curve for the entire length, starting ever so slightly from the sharpening notch at the back, and gradually increasing to the tip. Nuthin' but belly, which will make this a really nice slicer.
With this size knife, I'm not going to be doing any batoning. So, for me, grinding off the "harpoon" isn't worth the change in balance and the loss of the handy indexing feature at the back of the swedge.
Speaking of the swedge, I noticed something different between the Harpoon and both the BK 15 and BK 5. On the latter two, the swedge is ground all the way to the tip. On the 18, the swedge stops about a quarter inch back from the tip, making it a little less delicate.
Two things I note about the 18 that should make it even better than the 15 for piercing/stabbing. First, when I lay the 18 straight on top of my 15, the 18's edge and spine intersect at an even more acute angle than the 15. Second, the point is dead-on in-line with the axis of the handle, as observed by taking a straight edge and laying it over the center of the front and back handle scale fasteners.
Out of the box, I always tune-up the factory edge with a diamond Lansky set followed by some diamond charged strops down to 0.25 micron. I noted the factory secondary bevel appeared to be greater than 30 degrees per side. I profiled down to 25 dps. The 1095 Cro-Van didn't object at all to the tune-up, and polished quite nicely.
The sheath is a home-run. Nice solid lock up. I'm not getting any of the rattle that others are reporting, but I'm not shaking it enough simulate Marcus Luttrell bouncing down an Afghan hillside, either.
I find the attachment for the belt loop to be a very nice thumb ramp for withdrawal. And ditto on the spring strip in the snap closure that holds it open when unsnapped. That is a VERY nice attention to detail. Kudos to whoever decided to add that.
In case anybody's wondering, the slot in the sheath is just wide enough to accommodate the small Tec-Loc. I would love to see some different aftermarket options to accommodate "scout" carry. The reason my Tec-Loc is floating around in a drawer in the garage is that I prefer belt loops for a slimmer profile. I used to carry the 15 in an AZWELKE taco style with a single "Pull-the Dot" belt loop IWB at 8:00. A similar rig for the 18 is now on my wish list.
Over the next couple of weeks, I'll put it through some tests for light work in the yard and the kitchen head-to-head with the 15. From my first impressions, the only thing that I anticipate the 15 having a slight advantage over might be push-cutting thin slices of tomatoes due to edge geometry. However, that might be offset by the different ergonomics offered by the 18's blade shape and lower cutting edge relative to the fingers. Time will tell.