According to the changelog, OS X 10.11.5 will introduce under the hood improvements, various bug fixes, and security enhancements. This means that if you were hoping for UI tweaks or changes that were more aesthetic in nature, this is probably not the update that you were hoping for (but you should probably update all the same).
Now if you are one of those who have been affected by the system-wide freezes that was the result of the OS X 10.11.4 update, it does not seem like the update has fixed it. As pointed out by some, the development of OS X 10.11.5 begun before reports of freezing had surfaced, so unless Apple threw in something at the last minute, you might be disappointed.
According to the posts on the MacRumors forums, some users have reported that after the update to 10.11.5 that their systems have stopped freezing, or at least it occurs on a less frequent basis. Others have reported no changes, and some are now claiming that the freezing is app-specific, such as with Safari. We’re not sure if this is the result of OS X 10.11.5 or it could be a separate issue, but that’s worth taking note of if you are planning on updating.