Understandably, Lions fans are underwhelmed by Marinelli (perhaps because his name sounds too much like Mariucci). And understandably this is not the best situation for Marinelli to walk into. But Marinelli's window to coach a team was quickly closing and this offseason looks like the best chance for an assistant coach to make the jump to head coach. Marinelli had to accept this job if he wanted to be a head coach.
All in all, this is not a bad first coaching gig. Lions fans already have low expectations, so Marinelli does not have to win the Super Bowl in his first year. The Lions have enough talent on both sides of the ball that some smart free agent moves (quarterback, offensive line) can turn this team around. And Matt Millen has earned the trust of the owners, the Ford family (although I'm not sure how he did that), which means you can expect a good deal of stability if Marinelli can win some games next season.
And on side note, it's interesting to see the Bucs slowly take over the old NFC Central division. You have former Bucs coaching the Bears and Lions (Lovie Smith and Marinelli), and a former Buc running the defense in Minnesota (Mike Tomlin). I don't know who's left to take over Green Bay, but it's good to see the former punching bag of the old division teaching their old opponents how to play.