Picked up a 1080p 144 Hz (180 w/OC) monitor not too long ago - I think there was a thread on here previously about the new upcoming 27" 240 Hz model and I imparted my wisdom in sticking with 1080p - and I do have to say that, as someone who has lived at 60 FPS (solid) for so many years there is simply no turning back. For the record, I tested 60 vs 144 on this monitor, 144 vs 160/170/180, V-Sync (G-Sync off) vs. G-Sync, and many combinations thereof. Without a doubt, adaptive sync (not to be confused with adaptive vsync) is incredible and gaming in the 85-120 FPS range is unparalleled. I can't ever go back. I do think a 1080 Ti is minimum if you want that at 1440p but quite frankly I think 1080p will suit me for 3-5 years before I revisit the question (and I have a GTX 1080 which I will update at least once in that time frame). Actually, the 24" size worked out well for me as it gives faster eye control, but I digress.
For those wondering I even compared Rainbow Six Siege side-by-side on my 24" IPS (60 Hz) and the difference was ridiculous. No comparison. It's not just the 60 Hz but V-Sync that is just unplayable now that I have that game running at a 141 FPS G-Sync cap (using settings to keep it in the ~120 range to avoid cap latency). Also, 1ms response vs 5ms especially when paired with my G900 (1000 Hz polling rate) mouse makes the gap larger. Now, many people point to RB6's 50-60 tick-rate (soon to be full 60) and say that higher FPS doesn't help there...nothing could be further from the truth as not everything is synced anyway.
The tl;dr of it is, going from a 125 Hz polling mouse (-7ms input lag), from IPS to TN (-4ms), adaptive sync from V-Sync (-20 to 30ms), and 60 Hz to 120 Hz (where I peg most games, -8.33ms) makes a massive difference. Of course, games are just plain smoother, too! Yet it's one of those things that you won't notice until you try going back. After playing a weekend of RB6 at 120+ I went back to my old IPS and the latency was simply atrocious. Really, it felt like 30 FPS used to feel going from 60 (ah, LA Noire launch memories). Now, there's a lot of nuance to adaptive sync I won't get into here, but I suggest reading the Blur Buster's G-Sync guide.