I've been a part of guilds before, an officer in one once upon a time, and sometimes they're nice, but I usually end up spending too much time doing things for/with them and not enough time doing the things I like better.

But it really does depend on the type of player. If group-play and social interaction are big for you, a decent guild will go a long way. If, like me, you're more into soloing and exploring and doing what you want when you want to, then there's nothing wrong with going guild-free.