Buy this. It is great. If you would like to know more read on.
No need to get some fancy 11 speed. Use the derailleur, shifter, freehub, chain, and cassette you already have. Just slap this Giant Cog on and you be partaking of that sweet wide range nectar.
Easy to install. As long as you have a cassette lockring tool and a chain whip. If not take this and your bike/rear wheel to your local bike shop and they will be able to install it. Make sure your rear derailleur will accommodate the tooth change from your smallest cog to your biggest cog. You may also need to lengthen your chain after installing.
To install the WTC Giant Cog you have to remove either the 15T or 17T cogs from your cassette. I would recommend you remove both and replace them with the 16T cog from Wolf Tooth components. It is pretty cheap and will give you a much more consistent jump between gears. Make sure your cassette is compatible with the Giant Cog. Also, make sure you buy the correct Giant Cog for your cassette brand.
The cog functions great. Shifts smooth and feels like a normal gear, just much lower. I got the 42T. I would recommend this size. If you are looking for the lowest gear possible, go with the 42T. You won't regret it. I used mine with a Shimano 11-36 XT Cassette. The XT and XTR cassettes are compatible without modifications. I think you can also use it with the SLX cassette with a slight modification. Check out wolftoothcomponents.com for more information.
If using a Shimano Shadow + derailleur like I did (I used an SLX derailleur but it works with other Shimano derailleurs), I would recommend getting the GoatLink by Wolf Tooth Components and Lindarets. See their website for compatibility. Google can help you get there. It replaces the little link on the derailleur that attaches the derailleur to the hanger. It improves the position of the derailleur improving shifting. With the GoatLink shifting into the 42T cog felt just like shifting into any other cog. I have heard that Sram derailleurs work without any fancy links, but I have not tested one to know for sure.
I used this with a 1x10 set up with a narrow wide 30T front chainring. With the giant cog I was able to climb anything on Slickrock. It also made my local trails in Upstate Utah much more rideable. This single product significantly cut down on the number of hike a bikes and make my legs last much longer. I'm being serious.
With the price of the Giant Cog, the 16T cog, and the GoatLink things start to add up, but it is still much cheaper than converting to a 1x11 setup. Much much cheaper and easier. I am a converted to the wide range 1x10 gospel and I want to spread the joy I have found with others. Get your range back while still living the simple 1 by life!