Is 120Hz a lot better than 60Hz?

That is entirely up to personal preference. Neither is "better" than the other.
I personally prefer a higher resolution over a higher refresh rate.

Really no such thing as 120Hz. It's a gimmick. It's all still 60Hz. What happens is you have 1 frame per second at 60Hz. With 120Hz, it'll show the same frame 2x a second. To the viewer it just makes for a little less ghosting appearance, and this translates to apparent smoother motion. Does nothing for details.

If your game play is mostly fast action games like gta:V etc, a 1080p/144Hz monitor makes sense.

If your game play is slower strategy, sniper, mmorpg games, then a 1440p/60Hz would be better.

This is WRONG in every single way. 120/144hz screens display 120/144 frames per second and do not double 60/72 fps. If you can not maintain those frame rates duplication can happen(even at 60hz). With a fast enough card a 120/144hz screen displays 120/144 fps. Detail is unchanged either way since it is the same polygon/textures being displayed and it is all about smoother motion. Showing each frame 2 times does not increase smoothness without some kind of interpolation(this is what 120/140hz TVs do)

It is personal preference for some and some users do not even notice the difference.

I think he's referring to TVs that advertise 120Hz, which do use frame interpolation and aren't real 120Hz displays unless the TV in question has 3D capabilities. Most high refresh rate displays sold today are 144Hz, with 120Hz mostly being seen on the few remaining 3D capable monitors after that fad died out.

Cant remember the last time I saw a 120Hz monitor, so since op referred to 120Hz, I assumed (maybe incorrectly) that he was referring to a TV. And that is what Tv's do, (led or lcd, plasmas are different) be it the 120Hz,240Hz or even the few 480Hz.
As I understand it, 120/240Hz Tv's don't need interpolation, that's only used on 60Hz Tv's because films/shows etc shot in 24fps won't divide evenly into that 60Hz, 2.5 and can't show a 1/2 a frame, so the frames are multiplied then divided for an even number. Not needed for 120 or 240 since both 24 and the occasional 30 frames divide evenly 5-4 etc.

Hi,
A bit of confusion it appears. Let me explain.

*First, NEVER use this feature on an HDTV for gaming.

(use only for sports. it's not even good for movies since it throws off the motion blur which is adjusted for 24FPS)

Your INPUT does not change. It's still 60Hz (even if a 30FPS video). The Video processing in the HDTV has to sample SEVERAL FRAMES to see how the pixels are changing over time (acceleration) then it artificially creates ONE new frame (120Hz) per original frame or if 240Hz it creates THREE new frames per one original.

The main PROBLEM is the significant LAG/Latency this adds (which is not an issue for normal video as it's non-interactive). When you move or press a button there is a noticeable delay. It's similar to using VSYNC when gaming (it buffers a frame to synch with the monitor/HDTV) only it's even worse due to the fact it has to buffer several frames and then process those frames before finally creating new ones.

Not only should you NOT use this feature, but you should also see if there is a "GAME" mode which means minimal video processing. You may also be able to manually turn down or disable some video processing settings for that specific HDMI input if there is no game mode.

Other:
There is a feature coming called "FAST SYNC" (NVidia) which works if you can create at least 2x the refresh. For example, on a 60Hz monitor/HDTV if you output at least 120FPS you will still only draw 60FPS to the screen (some frames are dropped) but you get less LAG. Responsiveness is faster. It works because the monitor draws the very latest frame created and not the slightly older one that gets dropped.

Fast Sync does work, I've tested it.

Another option to enable is "Adaptive VSYNC" which toggles VSYNC ON and OFF. If you create at least 60FPS then you get locked to 60FPS and synched to the refresh (to avoid SCREEN TEARING).

If you drop below 60FPS from the GPU output then VSYNC is now OFF which causes screen tearing but this avoids the STUTTER causes by the varying frame times (if you miss a refresh then the same frame is drawn again so you end up with multiples of 1/60th second... different frame times in the same second cause a micro-stutter).

*You can enable via NCP-> Manage 3D settings-> add game-> ... save

A good rule of thumb for settings with Adaptive VSYNC is to tweak game settings so you maintain 60FPS (60Hz monitor) at least 90% of the time. If screen tear becomes too frequent then go back and drop a few settings such as 8xMSAA to 4xMSAA to reduce the time below 60FPS.

(I don't set it globally because it causes issues in some applications such as causing SCREEN TEAR in game videos unless they are 60FPS)

For 144Hz game monitors (that aren't GSync or FreeSync) there is a "Half" option of Adaptive VSYNC which drops you to 72FPS. It's actually a very good choice since 144FPS can be hard to achieve at least 90% of the time even with a high-end PC.

Summary:
- motion smoothing BAD due to added latency (video processing time)
- FAST SYNC coming
- Adaptive VSYNC per game