With the Mariner's current budget, I can think of a lot worse ways to spend their money. They locked up the best free agent available and one that will draw in the fans - a problem they needed to rectify. And now that they've added Cory Hart, they should be in much better shape than in the past.
I think they still need a few more pieces, but they do have a lot of budget remaining so we'll have to see what they do.
My only criticism is the length of the deal. 10 years if far too many. But, if he a Felix are the face of the franchise for the next 10 years, I'm okay with it. They could always trade him away later, a la Detroit and Price Fielder.