Mind Control is useful for certain circumstances, mainly buffing you or your party or for CC purposes - and in pvp for walking people off of high places :P
The basics:
You will see through that mobs/players eyes and be able to control him for a certain amount of time.
Will last upto one minute against mobs and 10 seconds against other players.
It has a 20 yard range so you have to be careful you don't walk the mind controlled mob out of range.
Ranks 1-3 allow you to control a mob upto 2,3 or 4 levels above you respectively.
Your controlled mob has a debuff which slows its attack speed down by 25%.
Once controlled you will get a bar similar to a pet bar with a list of abilities you can cast.
You have no control over your own character until MC ends or you cancel it.
In soloing I've found it most useful when taking on several mobs I couldn't otherwise, or just for a bit of fun. If your controlled mob dies while you have it controlled you won't be able to loot it, so if you are bothered about the loot release it a little before it dies and finish it off. It creates quite a bit of threat so use it wisely in group situations, especially if you're the healer, make sure the tank is aware you will be mind controlling - though you'll only usually be doing that for buffs while healing.
In instances it can be useful to give you and your party buffs or as a limited form of crowd control for shadow priests. Wowwiki has a useful list of mobs and the spells/abilities you can cast while they are controlled, though this is mainly for instances -
Mind Control Edited, Feb 29th 2008 1:20pm by cococj