^This.

7800 is IMHO one of the best groups ever manufactured. If you asked me what the best 10 speed group is I would pick 7800 time and time again.

7900 blows chunks so bad it hurts. The shifting is horrid at best and the design of the levers lends them to horrible intrusion by sand/grit/etc

7800 will always demand a great price in the after market. Just like the old 8 speed 7400(?) does. 9 speed not so much.

Now DA 9000 - they fixed a lot of issues when they came out with 9000. It's a solid group for sure and almost back to 7800. It is on my short list for 11 speed recommendations.