Towards the latter years of my undergraduate Gerstein took over from Robarts as my study library of choice and looking back I don't know why I never picked it in the first place.
It's so well kept inside with all the readings room having large windows to daydream your study times away. You see squirrels frolicking in the grass while protestors marching on Queen's Park are an equal,y common sight (and then you go on twitter and find out what all the fuss is about).
The people that stick to Gerstein almost seem to have a cult-like community (at least it was so in my day) with each level of reading room having similar types of people that favour it. But when exams come all loyalties are laid to waste in the quest to find empty seats.
They have a good short term loan section which I used towards the end of my undergrad instead of buying the textbook for a class or two and for the most part availability was good. But come exam time everyone migrates on to libraries so any resource that's offered is stretched to the limit.
Their small group study rooms are good if you want to meet to discuss group projects like poster presentations. They have computer terminals on each level although most don't offer seats, so it's more conducive to look up what you want and keep moving. Wifi is reliable, they loose out on a star thanks to the lacklustre food service available which unfortunately is a UofT wide pandemic (but I hear it keeps getting better).
It's not the prettiest or quietest library on campus, but it does a great job in finding that nice middle ground so that it attracts not only life sci students but a lot of undergrads and grad students alike.