Everywhere I read, Choice items seem to be the hot item. Why is that? I know the boost it gives is substantial but I really hate being locked onto a single move. Is the constant switching really worth that boost?
Sets that use Choice items usually make the set with the Choice item in mind and use it for specific targets. A lot of thought is put into putting a Choice item on a Pokemon, it's common but it is definitely not something you just throw on without thinking about it. I'm mostly going to be talking about Black and White 2 (Generation 5) OU when I bring up examples (didn't really play X and Y OU) but they should be mostly applicable.
The boosts given by Choice items are very significant and sometimes required for a set to work. For example, Tyranitar is generally very slow, but with a Choice Scarf, he can outspeed very fast Pokemon like Latios, Latias, etc. and hit them first. That's a big difference. In Generation 5 a lot of teams had a fast Pokemon with a Choice Scarf known as a revenge killer; this let them take out a Pokemon that would otherwise sweep them. In this case, the speed was really important and they only needed to use one move anyway. Keldeo with a Choice Scarf was very popular because it could outspeed a Salamence who had used Dragon Dance. Only being able to use one move was not a big problem, because it only needed to use the move that could kill Salamence.
Sometimes you would use a Choice item on a Pokemon that would switch anyway. For example, Choice Specs Draco Meteor. Usually you'd switch after using Draco Meteor anyway due to the -2 you get. So if you're going to switch anyway, having to switch because of Choice items is not a big deal. Another example is U-turn or Volt Switch which would force you to switch after use.
Finally, sometimes the Pokemon needs the Choice item because they are not strong enough on their own. Back to the Generation 5 example, Choice Band Tyranitar could 2 hit KO Skarmory with Stone Edge, if it had a Choice Band. If it didn't, there was no chance it would ever beat it. Pokemon that mainly use priority moves like having that boost too because priority moves are generally weak.
Basically, Choice items are used for very specific situations that are generally worth not being able to use other moves, or in some cases where you wouldn't want to use other moves anyway. Each Pokemon that uses a Choice item has a very good and specific reason for doing so, so rather than asking why Pokemon use Choice items in general, a better question would be to ask why a specific Pokemon's set needs to use a Choice item.
Hope this helps and clears things up a bit.