Compared to the other UofT libraries (UTSC), I enjoy this one the most. it has a decent amount of space to study and great selection of books (very easy to skim through it).
Although it looks quiet scary from outside, and well inside it's somewhat intimidating also; Its pretty good, quiet and clean.
Computers on the first floor are mostly crowded however study rooms with decent computers are available. Get here early if you're looking for a seat or mid-afternoon past 5:30pm.
Avoid during reading week, exam week as well...
It does have a food court and a Starbucks that closes at 7pm on weekends. So no big need to starve while you're studying. It also has a decent cafeteria with plugs a nice tables if in need for study groups or want to eat and study at the same time.
Security is nice, perhaps not during exam period: past 10pm security checks for IDs.