It's never easy to trade a player as good as Cliff Lee, but considering the position the Mariners were in, M's blog Lookout Landing thinks they did pretty well for themselves:
As for the Mariners, this rare intra-divisional megatrade brought in good, long-term talent, and good, long-term talent is what a team in our position needs. It hurts. It does. You're never prepared to lose a guy like Cliff Lee, even when you know for absolute sure that the moment is coming. It's going to hurt to watch him in another uniform. A rival's uniform. But Smoak's a huge piece - far better than anything the team gave up to get Lee in the first place - and all in all, I'm probably about as happy as I could've been about a deal that sends Lee away.
Lee was of no further use to the Mariners this season, and Smoak certainly satisfies their desire to land a Major League-ready young bat.