Metal Roofing

I need a little info, please. we are looking to start installing residential metal roofing shingles and standing seam. we are in the middle of becoming certified and need to know others opinions, besides the distributors.
Questions we have include:
-compared to asphalt shingles how much more time consuming should installing metal shingles be?(need this for cost analysis and what to expect from our guys)
-the warranty seems great, but what is contractor experience say?
-what are the things to check at the end of the job to avoid any call backs? (meaning the little areas your employees may screw up, and that you should check, but may be hard to see by a simple going over of the roof)
-we are in northeast ohio and need to know if these systems can be installed year round?

any answers will be appreciated

I have been using the Decra metal shingles coated w/ granules they seem to be a great shingle. They are screwed to the roof and they have a 50 yr warranty.