It makes you immune to crowd control WHILE the shield is up, while also decreasing the duration of the shield.

If the damage pops instantly, it might be because you press the key for just a bit too long (i did that too when i began to use it), game then acts as if you pressed it twice, try to TAP the key, just once, your shield should remain. With perks, later on, the shield becomes much tougher if you have some block chance OR you have a perk which also creates a decoy, which can help a lot in maintaining the shield for a while too! Also, i don't know if it's because of my build / weapon but the damage when the shield breaks can be pretty BIG!

Hope this helps your concerns. Cheers!