I've been using the 70-300mm IS USM (non - L ) for about 8 years now and I'm happy with this lens (90% of the photos I've uploaded were taken with it). I'm going to Uganda in a couple of months and thought I might upgrade to the 100-400mm L, and possibly get the 2x Converter too.
A couple of weeks ago I had the opportunity to try out both on my camera, but was disappointed by a number of features - most significantly the push/pull aspect of the lens. It was very unwieldy because the bulk of the weight is at the far end of the lens. And the lens is more than double the weight of my 70-300mm. Adding the 2x Converter not only made the weight even greater, but the end of the lens was even further from the camera. On the rotate-type zooms (like to 70-300mm) the weight of the lens is right next to the camera and is easier to handle.
Plus, the autofocus doesn't work when the converter is added, and the focussing ring is at the end of the lens. Very difficult to hold steady by hand (actually, vitually impossible!), and the light level is decreased meaning a minimum of F11 when extended to 400/800mm.
Without the converter it was easier, the autofocus was very quick, and the quality of the images was improved upon my current lens. It's obviously a very good lens, but not for me, I'm afraid.
However, the improvement in the quality of the images has convinced me to get an L-series, so I'll probably get the 70-300mm L. The reviews state that it's sealed (the 100-400mm is not) which will be handy when I'm trekking the Mountain Gorillas in the rainforest. And the comments in the above posts has convinced me.
Hix