OK, here's my two or three cents:
Personally, I've heard/read two competing theories on garlic: 1) It stimulates a fish's appetite, and thus, helps to naturally bolster the fish's immune system. 2) Garlic seeps back out through a fish's pores and onto the skin, making it an undesirable host for Ich/parasites/diseases.
Both theories seem logical to me, esp the 2nd one. If you've ever been around someone who eats lots of garlic, it's enough to drive anything off. I use garlic from time to time, but just for the hell of it. Honestly, I've never cured a fish of Ich using just garlic. I might start with garlic but it always ends with copper, for better or for worse.
The bottom line is this: While garlic might not help, I'm pretty sure it doesn't hurt. So while I don't advocate putting all your stock in it, if it seems to work for you, keep right on using it!