I hate that you can't tell if one of your allies just garrisoned a tower or the enemy did. The purple doughnut that appears around the base of the tower should appear green / red instead to denote whether it is an allied or hostile garrison.
It's a great summoner because it lets you dive turrets and greatly reduce the damage you would have otherwise taken. Not to mention all of the defensive uses; the range is amazing for clutch saves). Since the game revolves around capping nodes, sometimes diving a turret is your only option. The only problem I have is I often find when I take it in solo queue and don't have time to communicate to my allies that I am going to use it, I will dive in, use garrison and my allies wont engage with me realizing I have just given us such an advantage.
TLDR: Please Riot, consider giving Garrison an Offensive vs. Defensive Animation (Green / Red glow instead of purple)