I've been here about a gazillion times, and have tried numerous other izakaya places in Toronto. I'm just going to say it, Guu is still the best.
Also, original Guu location on church is better than the sake bar location on bloor.
Fool proof dishes:
baked oyster, tuna tataki, black cod, kerage, mushroom and cheese bibimpap, beef tongue, and almond tofu.
Only thing I don't like is that it gets way too loud. I can barely hear people I go with. So I usually just sit there and eat. I suggest sitting on the patio in the summer.
And in terms of getting a seat. They never used to take reservations (I'm not sure if it's changed), but come at 5pm (when they open) or 9pm (last call for food is 10pm) to get seated immediately. 7-8pm you'll be looking at at least 1 h on Fri/Sat/Sun.
Happy eating.