Water filter

These days no household could possibly be without a water filter. So, onwards my hunt for a water filtration system for my house... To date, I've heard of four brands, Elken, Diamond, Coway and Oasis. O ya, and Amway (although I heard less of this lately). I'm most familiar with Elken, since that's what my parents have at our house, but that system is rather bulky. The rest I'm too familiar with. I've heard of a few unsettling things about Diamond but I've not talked with any degree of seriousness with any agents. Coway, I first heard of it during the furniture fair in PISA a few weeks ago. Oasis, there is that big billboard on the mainland side of the Penang Bridge. No additional info yet. Still no decision...