How well do you think iOS 6 will run on an iPhone 4?
I tried running iOS 4.0 on an iPhone 3G (came with iPhone OS 2) which didn't go well, necessitating a downgrade. Also Leopard on a PowerBook, (came with Panther), subsequently downgraded as well. Both devices were good with the 1st upgrade (iPhone 3G upgraded to iPhone OS 3, PowerBook upgraded to Tiger).
I'm afraid that the same thing will happen with my iPhone 4 which came with iOS 4, upgraded to 5... for upgrading to 6?
I feel like the best option is to wait and see how the community receives the update and then go from there. Unless you are very comfortable downgrading, in which case by all means try be an early adopter.
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So I'm optimistic that it'll run well on iPhone 4-era devices (my phone is a 4S, but I have two 4th-gen iPod Touches in my household).
That said, I'd second TgD in saying to give it a couple weeks after release and see what the community says then.