EF-S 55-250 STM; worth the upgrade?

Hoogineer wrote:

I don't think my 55-250 IS II buzzes very loudly when IS is engaged,

Perhaps buzzing isn't the best word to describe it. It's like a low ticking sound. I don't think it ever recorded on video on the few occasions I did use it for that but I found it disconcerting when looking through the view finder.

but it is really noisy during AF. It also has a horrible tendency to hunt for focus, focusing in and out, especially when the subject is noisy (e.g. a bird in the trees), even when using a specific AF point. Granted, this might have more to do with pairing with the 600d, which only has the single cross-type AF point in the center.

The AF was hit and miss for sure. Sometimes a little adjustment in AF point positoin would fix it and sometimes not. The STM just feels a lot more reliable in that regard.

Personally, I think IQ on the 55-250 IS II is sufficient for my amateur purposes, but I really would love better AF performance (although IQ looks much better on the STM version according to The Digital Picture).

The IQ is definitely better. It is sharp across the frame at almost all focal lengths and apertures below f16. Version I (and II as well I believe) had a couple of weak points, mostly above 200mm. e.g. to match the centre sharpness of the STM wide open @250mm you need to set version I at f8. Even then though, the borders are clearly softer. At every other aperture at this focal length, the STM is significantly sharper all the way to f16. The latter is important to me as my primary uses for this lens are telephoto isolation where I need f5.6 and macro where I need f11 or greater and the older version's IQ started falling apart at those apertures.