My group lessons were really helpful for a long time because there was only a few people in my freestyle group and the coaches deviated from the curriculum, so I was able to work on doubles, Prelim MITF and such.
But the last couple sessions I did, I felt like I wasn't getting much. If it wasn't my private coach teaching, I had problems. Other coaches would contradict what my private coach told me, mostly on axels and camels, or their technique just plain did not work well for me. Also if I worked on easier things I got zero instruction.. such as back threes, the stroking on pre-juv, lutzes- I know there's things to fix, but the group coach would just say 'great!' and move on to another kid.
Group lessons cost about $10 each which is the cost of a freestyle session, and I just told mom I thought I'd get a lot more out of an hour of solid practice than out of the group lesson. And now I've somehow managed to acquire private lesson time on that freestyle, so I've officially moved on from general group lessons, though my coach put me in a figure group. But I'm nervous about that one because I'm afraid whatever coach is there will put me to work on waltz 8 and forward figure, and I really need to work on backward figure, and I'll just sound like a complete snot if I go "Well I've already passed Prelim and I need to work on Pre juv." So that will be a dilemma, but I'll see how it turns out. The concentrated practice on the back figure for a full half hour is really going to help prepare for my test.
So I would say that once you start getting into private lessons, you start losing taste for the group lessons. But if it's not overcrowded with good coaches, they can be very beneficial.