Since I've been shooting mostly with Leica lately I use my Canon 5D more rarely, mostly for work. The one thing that is extremely hard to afford with Leica is a variety of lenses, not that the variety does not exist, it's a matter of price. Nobody I know shoot Leica lenses, so I could borrow some.
The one thing that is a lot of fun is once in a while to hop on a bike and ride around New York with 400mm in your backpack. 400mm is a lot of zoom! It really allows you to see and photograph things differently. Something that you would never get with 50mm.
I own Canon EF 70-200mm f/2.8L USM lens which in my opinion is one of the best Canon lenses that produces exceptional quality images with good color. On top of that I have Canon 2X II extender. It's a second generation extender that's quite old by now, but Canon offers the third one Canon 2X III. Seeing prices now surprises me that III costs $429 which is a little too expensive. I bough my second generation one for around $300 many years ago and now it costs only $240. But I believe Canon improved the quality of the extender, and there comes the price.
Shooting with 2X extender is a lot of fun. You get to see things that otherwise you can't even reach. The main downside is that your aperture drops from 2.8 to 5.6. It really needs to be super bright outside not to increase your ISO when using it. Another thing with super zoom is that you need to shoot with fairly fast shutter speed 1/500 and above (on the go with your hands), even preferably above 1000. The tiniest hand movement will reflect in one big blurry image. The second downside is that your image quality drops quite a bit, but that is in comparison to using Canon 70-200mm without any extender, which again has extremely good quality. But at the end of the day if your image ends up being fairly small posted online, this issue is somewhat irrelevant. The thing is I get 400mm for extra $300, without paying for extra large and heavy 400mm lens.