If you are getting a tv I'd say no. From what I understand, many 120hz tvs only accept a 60hz input and double the frames. This means you wont see any real improvement as the console will think its connected to a 60hz screen. Plus any smoothing effects will create huge latency and can be pretty bad for a game. You could search out a tv or large pc monitor that accepts the native input but I highly doubt we will see any 120hz games that aren't pretty simple. They will most likely be pushing for 1080p with maybe 60fps and focus the rest of the power on effects and other graphical areas.
That said the panel in a 120hz tv has the potential to be better due to faster refresh, therefore less ghosting. It could also benefit any non gaming activities the One wants to push for if you enable smoothing. In my experience though, it can have issues, especially if we see a rise of 30fps games again. I recently had to decide between a 60hz and a 120hz PC monitor for my consoles, and the 120hz had a strange slow motion effect that I guessed was from having to show a frame upwards of four times, but that's just a guess.
My advice (TL;DR): Just stick with a good 60hz tv or monitor and wait until we see any 120hz capable games. I highly doubt we will see any, and unless you plan to use it for other purposes, it might be a waste of money.