I have mixed feelings on it.
Pros: crowns for guild/ a place to fight when you can't do your map/ win goods quite often
Cons: Costs the guild for inactive players or even players who aren't participating in it
Tend to get disappointed with the prizes I win. One time I thought there was no bad prize, but the premium building turned out to be a premium decoration. I would have preferred goods.
I haven't gotten to level 3. Just surrendered the last battle of level 2. However, seems like it may be a bit hard to win level 3 since if I understand correctly, they are the same level as I am and their bonuses are rather high. Mine are a tad higher, but since I have to win against 8 twice with the same fighters according to what I have heard, I am not thinking it will be likely for me to win all of them. I guess I will see if I get to it, but am wondering if the prize will cover the cost of retraining so many fighters.