You know a place is popular when it's been a little over a month and there are already 11 reviews on Yelp.
I think the food is great. It's not mind-blasting spectacular but it's tasty. There are definitely some hit (ebimayo, kakuni, kakimayo, almond tofu) and misses (gyu carpaccio and hotate carpaccio come to mind) but it's really hard to beat the atmosphere. One of the things I love about NYC is St. Marks and how all the Japanese restaurants are just absolutely bustling. It's great to have something exciting and lively in Toronto.
A few notes on the lineup. Yes it can be long. Yes you have to wait outside. However, I think that this is really the norm that most people face in other major cities. It just goes to show you how few restaurants there are in Toronto (I would almost say none) that have people willing to wait over an hour for. People are so not used to waiting that everybody is complaining about it. In places like New York, waiting an hour is normal. So if you don't want to wait, then show up right when it opens for dinner (don't know what time it is). I was also told that they will soon be getting heat lamps so that will hopefully make the wait outside a little bit better.
Another niggle about the restaurant is the terrible door design. Whichever genius (Bennett Lo) decided that a restaurant in a cold weather area shouldn't have a vestibule is an idiot. Luckily I've had the privilege of sitting at the bar (you should sit there, it's so much more fun chatting with the staff) and not by the damn door. But I would imagine that the door opening and closing would be a major pain.
Finally, I would say that the staff are really great at Guu. It may be just the 'newness' of the staff so they are really excited but if they can keep it up, it will be really fun. As I mentioned earlier, it's just so lively. The atmosphere is really what is 50% of this restaurant.