First, Fielder is an athletic HUGE dude. Not "fast" athletic, but not FAT either.
Second, he will be playing first base all the time. Not a part-time 1st base/DH (according to what he reportedly told Tigers before he signed.)
Inge is likely going to be replaced by Cabrerra, and when Martinez comes back next season there could be a whole different scenario to look at.
I think it's great that they picked up an exciting player that can hit like he can! If the pitching holds, and we can get a few more base hits and home runs (or even just the THREAT of either one!) Tigers could be a scary team to play for anyone!
The length of the contract is obviously a bother, but he's still young, and while he is admittedly a 1-dimensional player, that actually is in his favor for a long term contract, since you wont' have to worry about him losing speed or range.
Don't forget Boesch is coming back. If HE can hit as well as he ought to...