It would be easy to expect the 70-300mm to be very similar to the 17-85mm it was bought with; same aperture range, both image stabilised, both with USM focus motors. Unfortunately this isn't the case. The 70-300mm certainly has more of a budget lens feel to it, with more compromises made to keep the price low.
Initially bought to be used as a birding lens we quickly found that the 112-480mm 35mm-equivalent focal length was far too short for most of things we were interested in, apart from larger birds and mammals eg seals or geese. With an already slow aperture, adding a teleconverter was out of the question so we were very frustrated with the small subjects we were getting (particularly coming from a digiscope background where focal lengths in the 1000mm+ range are the norm).
The lens extends with focusing, and the front element rotates (a little disconcerting with a large lens hood on). The lens also lacks a true ring USM motor, so it lacks the full time manual focusing as found on the 17-85mm. Although we don't use that feature often it is very irritating having to click into manual focus to bring the front element back to the minimum extension when packing away.
This all adds up to a lens that both feels and performs cheaply, in a focal length range that wasn't particularly useful for us (short end covered by better lenses, long end not long enough). It was very quickly replaced by a Sigma 150-500mm which, although not a stellar lens itself, was a good improvement in nearly all respects.
The lens now has a new home and hopefully in the right conditions is performing well for them. On a tight budget it gets a reasonable amount of reach for a variety of situations, but not the ones we wanted. Don't avoid but do check out (slightly more expensive) alternatives if you want a cheap telephoto zoom.