My three-month-old ds has been on soy formula since he was about a week old; he seemed intolerant to breastmilk, so our pediatrician recommended Similac Isomil Advance Soy Formula. He has done beautifully on it. We've had absolutely no problems. I asked about other brands (after comparing costs), and he said that he prefers that one's vitamin and mineral content to other brands, plus it has DHA and ARA, which is good for brain and eye development.
The only downside is the smell--burps and spit-up are not pleasant!
Also, I was a soy baby, back when formula wasn't nearly as beefed up with good stuff as it is nowadays, and I turned out just fine!
Hope that helps.