I am in a guild and have done all the brawls.
I think the random guild pairings may not be the greatest idea. I know it will shake out over time with the tiers, but right now there's people with full sets of legendary cards playing against small guilds with actual Novice/Bronze players :/ Makes for a lot of one sides tournaments.
I do like that there's more reason to try to work together as an actual team, since before it was just a thing to do to get a discount/bonus DEC.