I've got the Alienware AW3418DW since about two weeks. Coming from dual PG278Q/PG278QR setup.
The screen is extremely responsive even against the TN panels (still have the PG278QR) and as shown in TFTcentral review.
Very nice experience with extra FOV and IPS colors, for now I would agree with those that say once you go UW you cannot go back.
I havent played FPS multiplayer extensively yet though, so maybe it's not the best for that, that's why I keep the PG278QR for now, just in case.
Everything is good with a GTX1080 and Gsync, no lags whatsoever.
You feel the difference between 100Hz and 120Hz, older Asus and Acer are 60Hz with maybe 100Hz overclock (many cannot reach 100Hz). I would not buy one of these older 60/100Hz monitors unless it was a lot cheaper than the new ones. They are 2 years old now.
All these UW panels have issues, the Alienware has not great contrast and on mine right side has slightly more yellow whites (seems to change on viewing angle), but 120Hz works fine, no buzzing and I cannot see scanlines at 120Hz.
I bought it for the responsiveness not ultimate image quality so for me it's fine, but if you are very picky about image quality I think you should stay away from any of these.