When I played, I leveled to max (60 at the time) on my first character without joining a guild. I eventually ended up having 5 or 6 70s, and only guilded one of them on the way up. I just didn't care for the idea of a guild. I was pretty accomplishment-focused, and didn't like helping guildies without getting anything for it. Not because I didn't like them (I'm not a mean person, really I'm not ) but just because when I played, I wanted to make progress. Anyway, I felt bad about that, so I didn't bother joining a guild.

I really don't feel like it hampered me at all, either. All of the "where is X" "how do I complete X" questions are more easily answered on the various database websites than by asking people. As for hard quests and instances, as long as you don't have your heart set on running a particular thing at a particular time, there are always groups to be had. I used to just do random quests I could solo, and if someone asked for a group member for an elite quest in my area or an instance or something, I'd join up. Even if a guild might have run through things faster than a pick up group, the time saved by not having to help guildies with things you don't need easily outweighs the slightly slower PUGing. After all, even when PUGs fail to finish, you still get plenty of experience and items. The extra piece or two of equipment that you might have picked up on a successful guild run isn't likely to last you more than a level or two anyway.

So yeah, it depends on the kind of player. If you're on mainly to have fun, play casually, chat with people, and not really get much done, leveling guilds are great. If you like gaining levels and making progress though, I'm not at all convinced that the average guild will improve things very much.