This is my initial review of the 55-250 STM. I will update it with images and more impressions as I use it through this fall and winter.
Why I purchased this lens:
I spend all of my photography time doing landscape photography. As such I went with the obvious choice in the last several years to use full frame cameras for best image quality. For the most part this was true, especially for night photography at high ISO levels.
Now that I am looking to add some wildlife versatility to my camera gear, I needed to figure out a crop APS-C kit that would work for landscapes as well. I already have a 55-250 IS for a t1i and it has a perfect zoom range but horrid image quality compared to the 70-200 L I used before. The problem with the 70-200 F4 L IS on crop for landscapes is that it is way heavier than it needed to be for a crop camera, and 70mm isn't wide enough on a crop camera for landscapes when paired with an UWA lens. Considering the price of my full frame lenses, this lens is practically disposable.
There was no lightweight and high quality EF-S lens in this range until the 55-250 STM was released not long ago. For landscape apertures, the STM version is near or equal to the L lens in image quality. 100% of my telephoto landscapes are stitched panoramas anyway, so any small differences are beyond any detection even when printed 10 feet wide.
Image Quality:
This is what most people are interested in when deciding to upgrade a lens. Quite simply the image quality is outstanding. When you factor the price, the image quality is out of this world. It's super sharp and most importantly it doesn't suck at any zoom setting or any aperture. That's the truly shocking part of this "cheapo" lens.
There is heavier vignetting than the 70-200 wide open, but evens out nicely stopped down. This is a 100% non-issue for my landscape work. Chromatic aberrations are well controlled, with a slight purple fringing (on t1i) in extreme specular highlights.
Flare control when pointed near or directly into the Sun is incredible. The 70-200L could only dream of controlling flare this well. All apertures and all zoom settings pointed into the Sun are pretty clean. No green blobs or laser beams. 
Color quality is really superb as well. The green and red channels don't seem to be altered much as with many other cheaper lenses I have used. I so far have not seen any need to apply color correction to this lens.
Build Quality:
Again. Canon hits a home run for the price. Unlike my other "cheapo" Canon lenses, this one doesn't wobble around like a McDonald's Happy Meal toy. Sorry, that was an insult to Happy Meal toys, they are built better than that.
Everything is solid and smooth. The extending lens barrel doesn't wobble at all and should be able to handle the weight of a landscape filter kit on the front without ruining lens alignment. This is pretty much impossible to avoid on the vast majority of cheap zoom lenses that extend, but the Canon is solid. Time will tell if it remains solid with more use.
The manual focus ring is a little less damped than I would like for landscape use, but I can see it working great for video use. I can just put a rubber band on it to hold it in place or to add damping if it is an issue. I've dealt with worse focus rings, so no big deal.
What it's like in use:
Everything works like you would expect a lens to work, except for manual focusing. The lens does not actually manually focus when you turn the ring. It does it electronically. Thus the camera must be active in order to focus. Focusing and image stabilization are both nearly completely silent. I have to put my ear on the lens to hear anything.
This lens also has a nice trick up its sleeve at 250mm. It can produce some pretty close-up images. With the image sharpness this thing has, it can be cropped a bit down to macro levels. This feature will come in handy when doing the rare extreme telephoto focus stacking. No extension tubes!
Conclusion:
If you didn't read any of the review, in summary, this lens is trash. Worst purchase ever. 
