So the faster the refresh rate, the better the HDTV, right? More frames look smoother, right? In theory: yes. In marketing: very yes. In practice: not so much.
Pulldown and the Film-Video Dance
To answer those questions, you have to understand two important things about video. First, you can't add detail beyond what is already in the source footage. Second, the source footage is never greater than 60Hz. When you watch a movie on Blu-ray, it's a 1080p picture at 60 Hz. The disc displays 60 interlaced or 30 progressive frames at 1,920-by-1,080 resolution per second of video. For movies that were recorded on film, the original footage is actually 24 frames per second, upconverted to 30 frames through a process known as 2:3 pulldown. It distributes the source frames so they can be spread across 30 instead of 24 frames per second. Those frames are then interlaced (combined and shuffled) to 60 "frames" per second to match the 60Hz refresh rate of the vast majority of TVs you can buy today. In the case of 1080p60 televisions, the frames are pulled down to 60 full frames per second, and both the players and HDTVs outright skip any interlacing step.
This is a time-honored tradition, because American TVs have displayed 30 (actually, 29.97) frames per second and functioned at 60Hz since time immemorial. It's not really a problem, because between interlacing and frame pulldown, the process doesn't attempt to add information to the picture. It's simply converting it to function on the TV, because it wouldn't work otherwise. 1080p60 is the current high-end standard for HDTVs, and no commercial media exceeds that resolution or frame rate. In fact, many movies on Blu-ray even turn the frame rate down and display 1080p24, or 1,920-by-1080 video at 24 frames per second, to make the footage look as close to film as possible. The various refresh rate-increasing technologies on HDTVs destroy that effect.
Going Too Far
Enhanced refresh rates like 120Hz, 240Hz, and various other speed-boosting features on modern HDTVs, on the other hand, push the concept too far. Remember what I said earlier about not being able to add detail beyond what's in the source footage? That's exactly what those higher refresh rates do. They interpolate data between each frame to produce additional frames. But the data in those combined frames can only be based on the source frames and whatever mathematical magic the HDTV is employing to figure out the middle ground. This technique can help reduce judder, or the jerkiness that manifests when displaying footage on a display that doesn't share its native frame rate (like, for example, a 24-frame-per-second film clip pulled down to 30 fps, then interlaced to 60Hz). Some plasma HDTVs can even reach a 600Hz refresh rate, which, when you consider that the source footage is going to be between 24 to 60 frames per second, is downright overkill.
Actually, this effect can produce a distinctly artificial, unnatural feel to video. Motion can appear too smooth, almost dreamlike compared with the films and television shows we've spent decades teaching our brains to enjoy. Action can seem just slightly sped up to the point of looking unreal, and it can take you out of the experience quicker than any judder. Indeed, to many videophiles judder is just as natural as film grain, and a subtle but necessary part of watching TV and movies.
Do You Need Super-Fast Refresh Rates?
When flat-panel HDTVs were in their infancy, they suffered from motion blur. LCDs in particular tended to display distinct blurriness during very fast on-screen movements because of "ghosting," or the afterimage left after the image on the screen has changed. LCD technology has progressed a great deal over the past several years, and now ghosting and motion blur have been all but eliminated. If you've purchased an LCD HDTV in the last two years, it probably won't show noticeable blur at its standard 60Hz refresh rate.
Tastes can vary, and you might enjoy the potential judder-reducing, motion-smoothing effects of an HDTV with 120Hz or 240Hz modes. But they don't add any actual detail to the video, and they certainly shouldn't be considered dealbreakers when you're shopping for an HDTV. Even if you get a set that supports 120Hz or 240Hz (or even 480Hz or 600Hz) video modes, you might want to disable them, and watch the video without any interpolation or judder-reducing effects.