Seems like a boom or bust type of pick. There's an obvious need in the secondary but there are concerns about Jenkins off the field. Still, he's a heck of a talent and this could be a steal of a pick if he pans out.
SB Nation's Lions blog, Pride of Detroit, explains the pick:
As tempting as it was to go defensive line in the first round for the Lions for the third year in a row and select somebody like defensive end Whitney Mercilus, I couldn't pass up the chance to improve the Lions' secondary. A year ago, when it was suspected that Janoris Jenkins would go pro, many people had predicted the Lions to take him with the 13th overall pick. Then he decided to stay in school and ultimately landed at North Alabama after being kicked out of Florida.
There are definitely several character concerns surrounding Jenkins, but the Lions have a very strong locker room, especially on defense. There are a lot of leaders on the team who would take Jenkins under their wing, and he is talented enough to select, even with the concerns. He has been compared to Asante Samuel, who the Lions nearly acquired last summer and were briefly linked to at the end of March. Rather than give up a few draft picks and a high salary to trade for Samuel, the Lions' best bet is to draft a player like Jenkins. He would fill a big need at cornerback and immediately improve the Lions' pass defense.
Janoris Jenkins 2012 NFL Draft Profile (via sbnnfl)