I've had my SG33 for just over two months now, and have really warmed to it. :)
It is as cavernous as the other reviewers say – it will comfortably carry everything I need for two weeks/indefinite travelling or a big shop, and I don’t think I will ever need to buy a bigger backpack. At the same time, it is still small enough to get allowed on as a carry-on bag and, if not stuffed to the brim, I think it will even fit under an airplane seat.
I would advise you to avoid the temptation to overpack when travelling for these reasons, but also because you could easily put enough it it to make it very heavy. For me, this bag feels like it will be at its best with 20L worth of packing in the main compartment, with the top compartment and remaining space in the main compartment left for bulky, light items, e.g. a hoodie, rain jacket, snacks etc. The top compartment itself is also pretty voluminous and will be very handy for quick access - umbrella, passport, hat, scarf, liquids for airport security, etc.
The quality of manufacturing is exceptional. I can believe the other reviewers when they say theirs still look brand-new after years of use. The look is unique and stylish, without being too flashy or over the top - it somehow manages to be both grungy and professional all at once!
In terms of things I'd change, ideally I'd want to use a water bladder with this bag while travelling/hiking, and am surprised this wasn't considered in the design (or even with the original Guide's…