Semi-off-topic but may be of some use. I was debating heavily between similar options (and was also heavily considering a 70-300 DO which I still would love to give a good test, especially on something like a 5DIII, etc., with it's great low-light performance, this lens can be found used for fairly cheap) -overall I needed smaller, lighter and faster as requirements. Ultimately I ended up getting a mint but used 135L which on my 7D gets me to 216 @f/2 and when combined with a mint, used, 1.4x converter gets me to 302.4 @f/2.8, when used on my full frame it's a regular 135 or 189 with extender which realistically is when I will likely just opt switch it to the 7D without extender for the extra stop of light, but would like to test more to see what practical difference in DOF and IQ is for times when I don't need the extra light.
It doesn't have the same ease of use/practicality as a 70-200 or 70-300 zoom lens would, but... combined with my 24-70 which is my main lens, I more or less have a decent portion of the same range basically covered (missing the 85mm and 100mm which if space ever allows, which currently it doesn't, would be filled with the 85L and 100L IS Macro), on top of this it's very light, fairly small, especially without the extender. The quality is quite good. It's a less obvious, black lens, and I hockey taped the extender to match and it has speed. At worst this combo is an f/2.8 @302.4mm, at best it's a 216mm @f/2. It's intended purpose is for night-time photo-journalistic style and street photography, allowing me to get in close while being far away. So far, with limited experience in testing this setup (I got it rather recently), it can get great results, even in extremely low-light, but is no-where near as easy to use as a 24-70 or any other zoom. I mean, it's a different range of photography, being tele, and it's a prime, but basically I'm just warning, if you want to try this setup for any of it's potential benefits, it's not the easiest style, at least not immediately.
Also, it lacks IS, which I make up for with shutter speed and raising the ISO. I'll edit this post and upload a shot or two from my first run with it in a bit.
Canon 7D, 135L @f/4.5, ISO 4000, Shutter 1/80, Effective Focal Length: 216mm Very weird glitch! (the bottom picture, the duller color wise of the two, is possibly a forum upload glitch??). The picture I posted here, has completely different color than as the file I uploaded. Even if I re-download and view it in the same image viewer it still has improper color vs. the original before uploading... Somehow uploading it to Canon Rumors changed the .jpg file? I'll put it on a different site so you can hopefully see the difference, very, very weird? Here, same picture, different upload site, very different results: http://minus.com/mnmBys4cH/1f
Canon 7D, 135L + 1.4x Ext MKII, @f/2.8, ISO 6400, Shutter 1/320, Effective Focal Length: 302.4mm