I have found as the years pass that tank bags have become my most critical piece of bike luggage. I put my wallet, my phone, earplugs, sunglasses and other sundry items in there that otherwise would be piled into my jacket pocket.
Being able to take the tank bag off and on quickly, keeping it off the expensive paintwork, and fit and finish are all very important to me. This is my first "tank Lock" style bag, and so far I really like the quick-detach convenience of it. SW Motech does a top-shelf job of building a bag that looks and feels of true luxury and quality.
SWM has not only the EVO line, but also a less expensive line known as ION, which is simpler in build and look. (BTW-- ION and EVO have their own line of rings, and they don't intermix)
Each bag is sold separate of the ring that your bike may require, but most retailers have a Bike fitment list. The RTW uses either a #126 or a #306, depending on whether or not its a Keyless bike or conventional keyed tank cap.