Is the problem that a big company owns it or that the beer quality will suffer?
Speaking strictly for myself, question #1 is "Do I like it?" If the answer is yes I may or may not buy it. If I haven't had it, I will probably try it.
My general guidelines beyond that are, is it a small company, and is it local? I want it to be locally made because it supports the local economy and water/glass is not being shipped all over the world, which pollutes and is wasteful. I want it to be from a small company because in my opinion that is important for competition, quality, and variety.
My local bottle shop stocks over 800 different bottles, and that is not including the many local breweries that don't distribute. I'm lucky, I can pick and choose. I've tried most of them, but when I go back for more I buy repeatedly based on several factors that include taste, style, brewery location, company size, are the owners cool or douchey, how well do I know the brewer personally, price, my mood, time of day, the list can go on and on.
The smallest and most local is of course my homebrew, and I like that plenty