User blog comment:StrategicMagic/The Most OP Item in League of Legends is.../@comment-5419795-20130406222204

Every 90 seconds, there are 19 minions (a slight simplification, but it makes calculation easier.) With the current Doran's ring, if you CS perfectly, you will gain 95 mana from the passive. That's just above 5 mana regen/5. It's good but not "the best buff in the game," especially since at 100 CS at 15 minutes, it goes down to a bit less than 3 mana regen/5, (with the flat regen only giving you about 0.15 mana per second more than the old one). It has some nice synergies, to be sure, and the burst regen can surprise unwitting opponents. But it's not a ubiquitous solution to mana problems, especially if you're behind and having a hard time CSing.