I have owned the the EF 70-300mm L IS USM for about a year now.
I bought it secondhand at a prize of around €1.000 - hardly used and as good as new.
It´s a fantastic lens. I use it with a 5d Classic for aviation and motorsports. I have even shot a few portraits with this lens, and the results were quite good.
Previously I used a Sigma 50-500mm, and I could not believe the difference I saw in image quality after taking my first pictures with the 70-300mm L lens.
Using focal lengths above 200mm, it´s a good idea to stop the lens down to F/8 - that will give you tack sharp pictures in this range.
Get to know this lens and the image quality will blow you away :-)
As others have said: Worth every penny. Highly recommendable :-)
I have since aquired a 300mm F/2.8 IS USM prime, but I wouldn´t dream of selling the 70-300mm anyway. The zoom is great for travelling: Very compact, light, built like a tank, weather-sealed and not overly expensive.
A few examples: