I was lucky enough to receive an MK25 as a gift last year, and since then I have been systematically trading all of my M&P's in for Sigs. The newest being the P320, Sig's striker fired, modular, polymer frame pistol. There are literally hundreds of posts on the P320 across the web so I'll save you the boring review and get to the pictures - interjecting with my impressions along the way.
I found it impressive that Sig gives you an OWB paddle holster. I liked that I could get to the range right away and run this from a holster without having to do any more shopping.
Note the "F-M" at the bottom of the backstrap indicating that this is the "Fullsize-Medium" frame. It's written out on the front right side of the frame in full as well (not sure why I didn't take a picture of that for you).
I throw Trijicon HD night sights on all my handguns first thing. Personally, my eyes struggle to focus on the front sight when the rear sight has defined dots as well, so I prefer something blacked out. The Trijicon HDs are nice because they have the anti-glare ribbing, but are inlaid with tritium to you still get three dots in the dark. Moreover, the P320 takes the same sights as the P228/P229, so don't worry if you can't find anything specific to the P320. They were a bitch to put on though, and the standard Sig sight tool (for p series) doesn't fit the P320 slide either. Unless you buy this beast from Brownells or own a press, good luck.
The trigger is easily the most impressive feature of the P320. Let me preface my next comments by saying that I use/used Apex triggers on all of my M&Ps, so I'm accustomed to awesome triggers. I also went back and forth between the P320, the Walther PPQ and the H&K VP9 before finally settling on the P320. I liked the PPQ trigger over that of the VP9 - both of which are miles ahead of a stock Glock or M&P trigger, but the trigger on the P320 won me over. It's hard to explain the feel other than to say it just feels solid. In fact, it wasn't a hell of a lot different than the trigger on my EDC Sig M11-A1. I guess I expected something that felt polymer, and this didn't. It felt Sig, but not old Sig; new, SRT-esque Sig.
But the thing that sets the P320 apart - as I'm sure you've already read all about - is that it;s built on a modular platform. The trigger assembly is what's serialized as "the gun" part, so you can quickly swap out different size frames - although I can't find any - and change out barrels and slides as well as calibers - which are available for limited application, but not available in all States.
I also liked the P320 over the PPQ and VP9 because the grip angle was identical to my other Sigs, so the handgun pointed natural in my hands. I know there are fellow #2A guys out there who collect a wide range of guns, but I like to keep mine in the same family. For me, the importance of every handgun I own feeling identical in my hands, having the same sights and operating the same way, is key. The P320 grip will be headed to Polytactical pretty quick for stippling, but it's really not too bad out of the box. I'd compare the textured areas to that of the Sig E2 grips. That said, it's lacking anything in the thumb area - which I like. You can also see from this pic that the mag release is pronounced, making it extremely easy to manipulate.
Another plus for me was the forward cocking serrations. Since this is .40 and I plan to use it as my 3-Gun handgun, I like being able to press-check easily, even with gloves.
My only real gripes with this weapon are that a) The slide is a little front-heavy so in addition to having the grip stippled, I plan on having the slide milled a bit on top and on the sides, and b) My left thumb rests right where the take-down leaver is on the frame. I guess I'm just use to my thumb riding in front of that lever on my other Sigs, Glocks, and M&Ps. For that reason, there's really no need for the forward frame stippling, but here's a rendering from Polytactical of where I think I'm going to take this gun before my first competition of the year: