I don't have one (yet) but the biggest thing is to make sure you understand the downsides of a TN panel. If you currently have one then you won't have a problem, however, if you have an IPS panel you're going to see a pretty significant drop in picture quality.
Here are some of the main points:
1) max resolution is usually 1920x1080, you won't find 1900x1200 or 2650x1440 unless you go to an IPS panel. Not a big deal if you aren't currently over 1080p.
2) most of the current selections are matte finish, if you have a glossy panel now you're going to be very disappointed as most of the anti-glare finishes are pretty heavy.
As for the 120hz itself, it makes a TREMENDOUS difference in gameplay. The picture refreshes twice as fast so that means there is significantly less lag between frames and even if you aren't above 60fps you will see a smoother picture. Most people don't realize how much framerate varies even if it looks to be stable because they display average framerate not realtime. If you want the technical nitty gritty a 60hz monitor has a lag of 16ms between each frame no matter how many FPS your video card is outputting, a 120hz cuts that in half to 8ms. You'll see the most benefit above 60fps, but even below that you can see some benefit because frames aren't generated evenly by the graphics card. IE: If you video card generates 60 frames in a second it doesn't always generate them 16ms apart, you might get half the frames twice as fast and half the frames twice as slow. That's why 120hz is ALWAYS better, because the frames that do arrive quicker will be displayed twice as fast as a 60hz monitor. Anyone who tells you 120hz only matters above 60fps isn't worth listening to because they don't understand the technology.
With that being said, running at 120fps @ 120hz is the way to go. For gameplay turning down the settings a little bit (1080p @ high instead of ultra) is better than maxxing out AA and the other graphical goodies. You'll enjoy the gameplay a lot more if your framerate is higher.
If you want a 24" (the sweetspot for 1080p resolution) I'd look at the Asus VG248QE, the anti-glare is pretty bad (not Dell bad, but still bad) but if you currently have a matte monitor you probably won't notice. It's a 144hz screen (i'd run at 120hz which is more reasonable for framerate, you can't really notice above 120hz anyway) and it has a good stand/bezel, is great for picture once you calibrate (for a TN panel) and has no input lag to speak of with a blazing fast 1ms response GTG. For gaming it is probably the best on the market. Early next year they are re-releasing it with the newly announced nVidia G-Sync too, it's currently around $279 and the re-release will be $399.
Edit: If you have a dual GPU (think dual 780 GTX) setup the other option to consider if you are ok with a bit of risk is one of the Korean IPS panels that you can overclock to 120hz. The QNIX or XSTAR 2710, the Overlord TempestOC or the Yamasaki Catleap 2B are all 1440p (2650x1440) IPS/PLS panels that come in glossy/matte finishes and can be overclocked. The risk is as follows:
1) most of the panels overclock well but it is not 100%. Most of the people I've seen post were able to get to at least 96hz (which is considerably better than 60hz) and a good portion got to 110-120, but you might not get one that goes all the way to 120hz without artifacts. It's not a huge gamble, but keep it in mind.
2) most of these panels are ones that were declined by QC by Apple, Samsung and LG. They are A and A- panels instead of A+, that means there is a slightly higher chance of backlight bleed or dead pixels. With that being said, at 2650x1440 even a couple dead pixels are almost un-noticable because you have a couple million extra pixels on the screen above 1080 and they are much smaller. The backlight bleed thing is a bigger concern, a lot of panels come with some backlight bleed because the bezel is cheap and they don't do a very good job of seating the panel. A lot of people have been able to open up the monitor and fix it with a bit of electrical tape to push the monitor into place more security, but if you don't want to open your panel you might end up with some bleed.
3) pretty much anyone you buy these monitors from has no warranty support. If it shows up DOA you'll have no problem getting a replacement, but if some pixels die within the first year you will have a harder time getting a replacement because these sellers are shipping them from Korea. This can be mitigated though, you can buy them on Amazon/Newegg if you watch for the right model to be available and if you really want to protect yourself you can buy a Squaretrade warranty for 50-60 dollars that will give you replacement cost back for 3 years if it breaks.
4) if your machine is not VERY powerful you will not be able to run your games at 1440p with a decent framerate. These panels only have dual link DVI and no scaler, so you cannot run in another resolution without having issues. I would honestly only recommend going this route if you have a beastly machine and the 300-400 dollars one of these would cost is completely disposable to you. If it isn't, then I'd recommend just going with the Asus I mentioned before, which is a totally reasonable monitor.
