Canon Extender EF 1.4X a great addition to your bag.

The Canon Extender EF 1.4X is a solid, hefty piece of equipment that adds significant zoom to (in my case) the Canon EF 70-200mm f/4.0L USM. The extender will cause you to lose an f stop but if your objective is to improve zoom capabilites then the tradeoff is worth it. I experienced little to no image degradation when using the extender. I also continued to enjoy the zippy zoom of the Canon EF 70-200mm f/4.0L USM with little loss of AF speed.

The nice thing about this extender is that you get a whole new zoom range without comprimising a lot of space in your bag.

I wouldn't recommend this piece for indoor photography as the loss of an f stop would hurt quite a bit, but for outdoor/nature/aviation photography it will give you the extra push if you need it.