Great route. Starting from Trieg the route is running through a forest. The road is small and can be busy. The way down to Martigny is awesome. Nice corners and great view.
As there is a motorway alternative to this pass there isnt that much traffic and as the surface is good mostly and there are loads of hairpins it makes a nice road for bikes (if a bit narrow in places) The straight section of road between Hinterrhein and Sufurs joins up the two most interesting bits.
The legendary Stelvio pass, an unbelievable selection of switchbacks on the Italian side of the Swiss border that has to be seen to be believed. It's almost always very busy and so is hard to get a clear run, but this is definitely a 'must do' for all bikers at least once.