Do it as soon as possible. After you get into the university, you will have immidiate access to spell making and enchanting altars. Unlike other factions, the mages guild quests don't really offer you many valuable leveled items for completing the quests. You will have a short time access to one of the most powerfull amulets in the game, though (you will be able to keep it by either exploiting a glitch or just not completing the guild questline)
One of the most useful things you will get is a unique mage's staff. This will be the only chance in the game to get a staff with a paralyze effect, which is normally most expensive spell in the game (also one of the best). The staff has 40 charges! Note that this effects is also disabled for weapon enchanting.
There are grat spells in every school of magic but they are mostly useful at expert to master levels.