I make rational decisions as a consumer, I think, but if you want to call it being a sheep, feel free.
Harvest Moon. I buy nearly every game I can get my hands on. There's a real reason for it, though: a lot of mainstream outlets don't review the games or give them only cursory reviews, so the only way to know what the game has to offer is to talk to fans, who almost always review the games positively, or play the games yourself. Sometimes, it pays off. I spent $60 on Animal Parade when it was rarer than it is now (I've seen it for $19 since), but I don't regret it because it is easily one of the best Wii games I own. On the other hand, I haven't really gotten into a single handheld title since HMDS, which was itself a glitchy game.
I'm not sure this is being a "sheep" so much as knowing what I like and being willing to take the risks. The payoff can be fantastic--HM64, Friends of Mineral Town/Back to Nature, Magical Melody, Tree of Tranquility, and Animal Parade are all exceptional--but at least half the time, there isn't going to be one. I buy the games knowing that full well.
I also haven't bought Save the Homeland or any pre-Friends of Mineral Town HM game because there is enough information about those games to let me know it might not be worth the trouble. I also haven't bought a console for a Harvest Moon game, so I've missed out on a few other titles.
Pokemon. I've never played a Pokemon game I haven't liked, so I've bought at least one game from every generation except the second. I own Red, Blue, Yellow, Emerald, LeafGreen, Diamond, Platinum, SoulSilver, White, and White 2.