Using a headset definitely helps when playing first person shooters, this goes all the way back to CS 1.6.
The biggest advantage of using a headset is that it gives you a much more focused sound projection and your ears give you a better gauge of where the sound is coming from. If the sound is emanating from your speakers, you won't be able pick up as quickly or as accurately where the sound is coming from.
The question of "is it really worth it"? Is somewhat dependent on how competitive you are or how well you want to play. How important is your Kill:Death Ratio? Are you more focused on trying to complete the team objective on the map?
If you're the type of player that wants every little bit of an advantage, a headset is definitely worth it.
As far as the second part of the question between a 5.1 headset and a stereo one, I've used a 100 dollar 5.1 headset, and a 10 dollar stereo headset (50 bucks the first time though, found OEM ones later for 10), from a sound standpoint, there isn't much difference when it comes to gaming.