Vikings fan here. Not looking to "talk smack", only to give my 2 cents on a WR who I have followed very closely for his entire NFL career.
First off, Percy is a heck of a weapon on offense. Whenever he plays you guys are going to be happy with him. His elusiveness with the ball in his hands is just silly. He is also tough as nails on gamedays, not shying away from contact at all, almost to the point of being reckless (when he was a Viking I used to worry about how long of a career he would have). His ability to run out of the backfield, both inside and outside the tackles, gives him that "gadget factor" where defenses always have to account for him no matter where he is on the field. He is amazing at getting YAC. I read someone on here say that he wasn't a fit on a Ponder-led offense, and that isn't true at all. With QBs like that who are best with short quick passes, Harvin is the perfect WR. It makes more sense to go for high-completion short passes and let Percy rack up YAC. He rarely saw deep passes downfield.
That being said, you are now getting your first taste of Percy World. He hates to practice. A lot. He also is nicked up a lot with nagging injuries. For as long as he's in the NFL, there will always be drama surrounding Percy Harvin. It's sad, because his attitude didn't really turn so poor until Randy Moss came back for that brief stint with the Vikings. Percy reportedly followed Moss around like a lost puppy and was enamored with him. Moss behaved horribly in his short return to Minnesota (ex: the lunchroom incident where Moss blew up and screamed that he wouldn't feed the team caterer's food to his dog). After Moss was let go, Harvin and then-coach Brad Childress got into a heated altercation, and Percy hurled a weight at Childress (although it didn't hit him).
In the end, I wouldn't be surprised if Percy ends up being fine medically and this was a way to get out of some practices. You can decide for yourselves how much that does or doesn't bother you. There was a time when those antics made Vikings fans say "Meh, as long as he plays on Sundays..." But over time, the constant off-field drama and prima-donna behavior wore on people and chased him out of Minnesota. It's not often you see teams trade away talented WRs in their prime. Especially since at the time we had a completely bare WR cupboard without Harvin. I don't want to see Percy fail in Seattle, the trade is done now and both teams got what they wanted. Hopefully it works out for both teams.