I prefer SW over Inventor personally and use both on a regular basis.
Yes you can import DWG/DXF files into SW. There is actually a DWG import dialog that lets you select layers to import, make it 2d or keep it 3d and so on. And SW uses blocks. I havent imported any blocks from Acad, but i use blocks all the time in SW.
SW can open IPT and IAM(never tried drawing files). What you end up with is a multibody part file in SW. Of course you loose all of your feature data but the physical parts will import. This only applies to current and older versions. SW2010 can't open an inventor 2011 file.
I really like the SW interface. Even though IV and SW is pretty similar in that aspect, the SW one seems to work better for me. Things like operations are a little different. For example SW has an Extrude and an Extrude-Cut (two separate functions), where inventor lets you select Add/Subtract/Intersect right in the feature. Apples and Oranges really.