I joined a study group my first year because I do learn better when I can talk about concepts with other people.
My study group did have its moments of distraction and socialization, but in general we got a lot done. In part, this was because our study group scheduled social events (e.g., we're getting together for dinner Saturday, so we can socialize then), and in part it was because the group's personalities meshed very well.
There was a huge element of accountability, knowing that I had to show up to study -- even if I didn't feel like it-- because my group was waiting for me. We had one woman who was the organizer, making sure to book study rooms. Another woman was the motivator, always hounding us to stay on track. Each of us brought flashcards, supplements, etc. All we did was run through hypos. And by the time an exam rolled around, we'd typically done hundreds of hypos.
Three members of our group (out of the five) graduated in the top 10 percent. A fourth finished not far behind. And the fifth is a joint degree student and will graduate next year. All of us swear that we could not have done as well as we did without our study group.
So, it really depends on the individual.