I succumbed and bought the EF-S 55-250mm STM version as Amazon UK recently dropped the price significantly.
Personally, I think it is just about the limit of handle-ability on the EOS.
Wouldn't want to use something much heavier or bigger hand-held.
But with the tripod collar on the EF adaptor it balances really well, and is light enough to stand alone even on my Video monopod
IQ seems good. Though I can't say it is a huge improvement on my original non-STM one I had but sold on to make way for this. But operationally it is significantly quieter (AF and IS wise) which is what I was keen on as I took several videos with my non-STM version and you could hear it furiously working away.
Will the EF-M version be much smaller? Well it can lose the weight and size of the adaptor of course, and they can perhaps shave a little of the body. But given my comment above about balance I am not sure that is necessarily a good thing if you lose the forward position of the tripod collar. It wouldn't balance on my monopod any more. (not that this will be an issue for a lot of folk)
I am working on a review for my site.