This is pure speculation and opinion.
The Mariners are mainly in the market for offense, but they could use one more starter as well, especially a starter on the cheap so they can acquire more bats. Not only is Colon a mid-tier pitcher, he has an excellent record in Seattle. Especially as of late he’s been crushing it in Seattle.
The Mariners also have a bat that is intriguing for the Mets, Nick Franklin. Even with the recent struggles with Miller in the lineup, their reluctance to recall Franklin to the majors suggests they are sticking with Miller. The Mets have been linked on the blogosphere with Franklin before, and I still think he would make an excellent addition to the lineup.
The question is if the trade is fair. From the Mets fan side, I believe yes this is fair. I would do this trade in a heart beat. From the Mariners side, I’m not sure. They are getting stuck with another large contract.