The value is in the mobile zone regeneration and the sustainability. So if you are casting him, cast him for that reason. If you are a 5 dice attack on a durable creature, cast a knight/troll instead.
Unicorn is great versus idol of pestilence and somewhat good versus poison zone damage (poison cloud, malacoda, plagued, ect). It takes some smarts to use the charge well, but when you triple down with lion savagery and holy avenger it is a lot of fun to hit things (9 dice +2 piercing for only 2 mage actions and 22 mana i believe).
To get the charge to work, keep the things your enemy is trying to kill one zone away from the unicorn and make the unicorn go later in the round. This may require your mage to do some kiting. Enemy Mobility hindrances are twice as bad for unicorn users.
You can also use plagued on a friendly unicorn for some fun zone damage. it lets you keep creatures in the zone without taking damage. I wish plagued were cheaper to cast on friendly creatures as well.
If you are the priest, he is maybe the forth most popular holy avenger. regen2 with 14 life and 2 armor may make him too big to bother attacking. If you are a holy mage, he is the only in-school regeneration option. If you are casting more than 3 creatures without regeneration(especially asserian defenders), think unicorn. Regeneration stacks well with guardian angels heal2 and soldiers high armor.
If you are a beastmaster, the unicorn supports mobile swarms the best and makes for an ok pet. Doesnt not work well with ragers like the gorilla. If you tanglevine the enemy and give him cheetaspeed, he can move in and out for the charge bonus.
I think that, and the posts before me, cover everything.