It’s good to read the helpful feedback, advice and thoughts in this thread. I’ll keep my input quite brief – though I could easily write pages about tele lenses, and my experiences.
At times there simply is no opportunity for a ‘Wow!’ or powerful photo, even with the best gear around. That’s when I aim to be satisfied with a photo as a ‘memory snap’. I think you had one of those opportunities… and interestingly I’ve had the same scenario with deer a few times when I lived in Europe (ie “cool some deer” – and taken a few photos before they ran off)– though never in my backyard!
My first telezoom lens with the Canon 100-300mm USM. No IS, soft and low contrast above about 180mm. Stopped down at 300mm with some careful post-processing I could coax decent image outcomes… but most time the lack of IQ (and lack of IS) was a real disadvantage. (Though to be fair I bought the lens new, on a good deal, and it is sharp & contrasty between 100mm – 180mm – and the USM motor was very quick and reliable). The 70-300mm IS nonL had just been released, but many copies were having the ‘portrait orientation issue’). Plus it was over double the cost, and I was on a limited budget back then (being an international charity worker).
Some years later (back in Australia) – I was looking at the Tamron 70-300mm VC USM or the Canon 70-300mm IS nonL to replace my Canon 100-300mm. I was leaning to the Tamron- though even its IQ is ‘decent’ (but not stellar) at 300mm from all the tests I show and reviews I’ve researched, IQ quite similar to the 70-300mm nonL’s. Right at that time, the 70-300mm L then was released – and I thought to myself ‘nah… too big, heavy & expensive’ – but I went into 2 shops with my 7D to try it anyway… and I was ‘sold’! The size / weight was a big consideration – it’s a very portable lens, with outstanding IQ – even at 300mm f/5.6. So I bought it – received a very good deal and a decent 67mm Hoya CPL to boot (every lens I have ever bought I have received some sort of discount / good deal).
If the 70-300mm L wouldn’t suit me, I would have bought either the Tamron 70-300mm or the Canon 70-300mm nonL. As I result of buying the L version, I sold my Canon 100-300mm (only lost about $150 for several years use) – and am very happy with my 70-300mm L. I like getting as close to wildlife as I can, and expect to keep using my 70-300mm L on high-end APS-C bodies for such sort of photography eg 7D and future 7Dmk__‘s It’s IQ, matched in a very portable body with USM and 4-stop IS make it a great set up for me.. which I can keep in my Lowepro shoulder bag – where it fits suitably with my 7D with 15-85mm USM IS.
Here is an example photo of a kookaburra taken a week ago, in the wild. At a photo outing that I organised for a group of about 8 friends. (which I do a few times a year – mainly as an encouragement to get involved in photography, and learn their cameras, learn new skills and socialise). The photo is slightly cropped (though note, this is definitely not at 100% yet). I have received lots of views of this photo, many friends are impressed. Others took photos with their gear (including 4xNikons DSLRs, another Canon, Sony RX-100, even an iPhone) and lenses ranging from various Nikon 18-55mm and 55-200mm, to Canon 100-400mm L IS USM. I certainly don’t mean to be proud – but this photo was by far the best of the ones taken by the group on that day, probably because I have most experience, and the 70-300mm L was the best lens (though I expect the 100-400mm L could have produced a close image, but the person who had / used that lens was new to photography – it was on his Canon 700D body).
Perhaps the best ‘bang for the buck’ lens of today is Canon’s recently released EF-S 55-250mm STM IS. It is a great lens for the price, producing very good images (though note- it won’t work on FF). It might lack the build quality, full USM, bokeh quality, micro-contrast and speed of some higher lenses, but for it's price - it's image quality is more than great: particularly raw sharpness. The 18-135mm STM IS is also a great all purpose lens I often recommend. Actually Canon haven’t produced a bad STM lens yet, eg the 40mm STM, 22mm STM and 18-55mm STM! Good on Canon for setting the bar high with STM lenses thus far.