I got to see G-Sync a few days ago when I got the Asus PG258Q, which taught me a lot of about G-Sync.
I wrote a review of my experience with the PG258Q in the General forum, so feel free to check it out if you want to know more:
http://forums.blurbusters.com/viewtopic.php?f=2&t=3287
Regarding G-Sync, I figured it was going to be a gimmick because my games run very smooth with V-Sync ON and I figured it can't get much smoother than that. However, once I tried games like For Honor, Mass Effect Andromeda Multiplayer, Rust, Tomb Raider (newest one), I was blown away by how smooth the gameplay was. What do I mean by that? For example, when I used to play Rust on my BENQ XL2420T with no G-Sync, the game would stutter when I would look around sometimes, and I figured this was just bad code from the Unity engine that Rust is using. Tomb Raider did the same thing, and again I figured it was bad code. Well, maybe it is bad code. But what I can tell you is that a game like Rust in particular, which suffers from a ton of stutters when looking around (especially in forest areas), almost NEVER stutters with G-Sync enabled. You get something like a 90% stutter reduction, and the experience is just so smooth and unbelievable. What's amazing is that even the fabled wide screen CRT, the Sony GDM-FW900, can't get motion as smooth as with G-Sync, because it doesn't support G-Sync. You will still see the stutters from Rust or Tomb Raider or other games on the CRT, even though the actual motion is beautiful and unparalleled when it isn't stuttering. So G-Sync is the very first technology when combined with a great motion LCD at 120hz+ that actually is superior to CRT in overall smoothness of gameplay. When I played Mass Effect Andromeda multiplayer, the explosions and laser effects were so smooth it just blew my mind. Smoother than even playing on CRT due to the lack of stutters (even if they are minor, they will show up frequently as you move and turn quickly through the map without G-Sync).
With that said, I noticed that when games run at 90+ FPS, G-Sync looks the best and feels the best. When I played Mass Effect Andromeda single player in a zone that was running at 50-90fps, the G-Sync actually made the game more choppy and sluggish. I had to turn off G-Sync to get a smoother game experience, because it felt almost like G-Sync was spreading the stuttering across everything like a bullet proof vest instead of taking hard stutter spikes like I'd normally get with boosts of faster motion. I actually prefer the hard stutter spikes with faster motion at lower fps than making the whole experience a more stuttery feel. I have no idea how some people online say that G-Sync is great for lower framerates because that certainly wasn't my experience. G-Sync (from my perspective) is ideal when you are about 90+ fps but can't quite max out the refresh rate of your monitor. In my case I was at 240hz so I was never reaching that maximum, but even at 120hz you can't always stay at a consistent 120 fps for every game, making G-Sync extremely useful for a consistent smooth experience in those cases. For lower framerate games you may just end up wanting to use ULMB instead if ULMB doesn't cause problems for you on the monitor in question. You may also want to use ULMB on games that have an fps that is constantly capped at your refresh rate.
While I thought the PG258Q was disappointing, the one thing I did learn from it is that G-Sync is an amazing new tool for those always looking for a smoother gameplay experience. It isn't a tool you use in all cases, but when you can use it, it is life changing to experience.