I kind of doubt the office floors are in that bad of shape. There was a pipe burst in 2001 that was the final blow to the occupancy of the building (and I seem to remember it was in the garage), but I doubt that you would find anything real exciting above the parking garage. For all I know, the office floors are intact. And, as Hayward and I have both said, you would almost certainly NOT be able to get above the garage.
As I said above, it's not like this building has been vacant for very long. I can't imagine it would be anything like exploring the MCS or pre-renovation Durant.
If it were me, I try my luck with this old school. It's a few blocks off of Saginaw right next to the interchange of I-69 and I-475 (south of I-69). Always been one of my favorite abandoned buildings in Flint.