I had the original 70-300 IS USM. It was a dog at longer than 250mm. I had version II. I agree with you that it is way better glass than the first version. I find it kind of amazing the you didn't mention the nano-USM focus motor. Blindingly fast, and nearly dead silent. Version II is a total upgrade over the first iteration. I sold version II to upgrade to the 55-250 STM. Yes, upgrade. It is smaller, cheaper, lighter than the II, and it is at least optically equivalent.
Oh, and one other point. All zoom lenses, whether they extend on zooming (70-300 II & L), or whether they zoom internally (all Canon 70-200) move air when they zoom. When they move air, they can suck in dust. Weather sealing the front element and rear mount has very little, if any, impact on that.