It was my favorite spell for a long time but lately I've been going with Ghost and Exhaust, sometimes Surge, depending on the champion.

The possible downsides of Garrison is it`s restricted effect, sometimes it`s damage is easily avoided by the enemy, and it`s effect is often wiped by the enemy`s Garrison, specially in big fights. There`s a lot of overlapping even between teammates.

Still a great spell, just my 2 cents on the possible downsides of it.

I like to use it 50/50. I can turret dive in I need to...and I play a lot of Amumu so when I use my Q, which sometimes puts me under the turret a lot. I also use it for 2v1 or 3v1's