Long story short… several years ago, I wanted a ‘telezoom’ to complement my Canon 28-135mm USM IS.
I bought Canon’s 100-300mm USM. Fast focus.

No IS

Sharp up to about 170mm. Not good at 200. Poor (unsharp & low contrast at 300mm, just ‘decent’ at f/11.).
I didn’t like any of the 75-300mm variants, and didn’t have the cash to spend on an L. I was quite happy with the 100-300mm for some shots, and with careful technique and clever post processing I could get ‘decent’ images from it, even at 300mm
The non-L 70-300mm IS ‘USM’ came along – but without ‘true’ USM, and it had a potential ‘portrait orientation IQ issue’ (connected with IS in that orientation). While it was overall sharp and had IS, it was more expensive. Plus the AF and build quality was a step down from my 100-300mm USM. So I passed on that lens.
A few years ago while I was ‘waiting’ for the update to the 70-300mm IS ‘USM’ (non L), I tried out the Tamron USD VC, which I quite liked. It had some good reviews, and had a bit better IQ and noticeably better build quality than the 70-300mm IS non-L, but AF was noted as a bit of an issue in some situations. I was thinking about that, or the 55-250mm IS – which was by all accounts sharper (the STM version wasn’t out at that time).
I happened to see the recently released 70-300mm L in store, and asked to try it. Took some photos with my 7D, analysed them at home, and anyway, I was sold. Bought it (got a good deal, and a 67mm Cir Polariser thrown in). Haven’t looked back.
I do see the 70-300mm IS non-L as being a ‘bit’ of an ‘in between’ lens now. In general for people on a budget, looking for a Canon telezoom, I recommend the great 55-250mm IS STM. It’s inexpensive, light and has good IQ overall. Build quality not fantastic, but not too bad for a ‘kit telezoom lens’. For others, depending on their needs I recommend the 70-300mm L (great IQ, very portable), 100-400mm L II (for a bit more reach), or the 70-200 f/2.8 L (if need f/2.8 – and don’t need 300mm to 400mm).
I can see the 70-300mm IS non-L is ‘due’ an update – especially as some have pointed out – to improve its IQ at the tele-end with the higher MP / more pixel dense APS-C DSLRs of today… And yes, it does not appear to be as good as the Nikon equivalent. So I read with interest this thread. Not that I’m going to sell my Canon 70-300mm L IS USM… it’s just too good a lens, and at just 1kg, and being a ‘compact design’ – I love it for all it gives to me!
Paul 8)