As a Broncos fan, this is great for Denver, even with the high cost of draft capital. They just couldn't afford yet another year of floundering around with some washout journeyman and/or project at QB while the rest of the division was B+ or better at QB. The fanbase was getting totally sick of that holding the rest of the team back. Now we got easily one of the top 5 or so guys in the league.
As for what Seahawks are getting... I assume they will take a long look at Lock to see if he is developable. Part of the rocky time he had in Denver was due to awful coaches, but he also seems to be lacking in read skills and is probably always going to be a turnover machine. Not sure if Seattle's staff can fix that.
Fant has talent, but attitude issues. Harris is a good throw in, but not young. Obviously, the use of the picks is what will most make or break this for them.
When you let a genuine franchise QB leave, the big hope of course is you can get lucky and come close to replacing that sooner than later. Its not impossible to get lucky, but way easier said than done given that those guys don't grow on trees.
