WoWDB

The elixir puts your character to sleep and creates a Dream Vision version.  Use an Elixir Master alchemist if you want to try for the biggest bang for your supply of materials. 

• It lasts for two minutes. (Four minutes if you are an alchemist and are able to craft the elixir. See  Mixology.)
• You will not aggro any monsters. 
• You are able to jump normally (used to be that you couldn't).
• You have a slowfall/levitate effect when you fall, as well as water-walking.
• You are unaffected by fatigue or breathe timers.
• You cannot interact with objects, mobs, or players.
• You cannot be attacked BUT your actual, non-Dream Vision you remains vulnerable to attack.
• Using any spells or abilities will cancel the Dream Vision.
• It will not overwrite any Battle and/or Guardian elixir buff.
• You will receive credit for explored areas.
• You are able to read comments in public channels of the area Dream Vision-you is in, but any comments you say are sent to the public channel of the area that your actual body is sleeping in.

Extremely useful for grabbing screenshots of mobs in their natural environment. :)  It's also great for checking for rare mobs in instances without having to clear as many mobs.  Handy for Horde characters (working on the  Universal Explorer achievement) with no stealth capabilities to get past the draenei guards protecting the Draenei starting zone... or enter the other faction's area in Dalaran (applies for Alliance characters, too).