I think a lot of you guys are confusing value with performance. It's not like Shimano purposely makes garbage products, so even the Sora-level stuff is going to work. I'm seeing a lot of posts defending the use of lower-level components, as if the perception is they are garbage or don't work properly. Obviously all the various level components are going to work, it's just a matter of whether a person can perceive the difference in groupsets. For me, personally, I feel a HUGE difference between the 6700 stuff and the 9000 stuff. But if I was going between 5700 and 6700, I might barely feel a difference, or none at all. And, there may be some people who won't feel the difference between any groupsets, regardless of its "level".

It's obviously up for debate the VALUE of Sora vs the VALUE of DA, but the quality difference is obvious.