The Guru of 3D published a review on the NVIDIA G-Sync Explored and Explained
A quote from the article:
NVIDIA recently announced G-Sync, a technology that will eliminate some pretty hardcore stuff, things we have taken for granted many years, screen-tearing and VSYNC stutters. For that you will need to dig a bit into your wallet though as you will need a compatible Geforce graphics card and a compatible monitor that has been equipped with an Nvidia G-Sync module. But once you have established that combination, you'll be nothing other then impressed -- that much we can guarantee! G-Sync, a technology that is named to be a game changer, yes G-Sync eliminates the problems that come with VSYNC (both on and off) versus what is displayed on your monitor. See, in recent years we all have been driven by the knowledge that on a 60 Hz monitor you preferably need 60 FPS rendered by your graphics card. This was for a very specific reason, you want the two as close as possible to each other as that offers you not only the best gaming experience, but also the best visual experience. Running 35 FPS on a 60 Hz screen with VSYNC on would be great, but you'd still see a hint of what I like to name "soft vsync stuttering".NVIDIA G-Sync Explored and Explained @ Guru3D
Then the hardcore FPS gamer obviously wants extremely high FPS, and for these frag-masters the alternative is simply disabling VSYNC. However if you have that same 35 FPS framerate on 60 Hz, you'd see visible screen tearing. Heck, this is why framerate limiters are so popular as you try to sync each rendered frame in line with your monitor refresh rate. But yeah, these are the main reasons for screen anomalies, and ever since the start of the 3d graphics revolutions (all hail 3dfx), we simply got used to these sync stutters and/or screen tearing. To compensate we have been purchasing extremely fast dedicated graphics cards to be to be able to match that screen refresh rate as close as possible.