NTFS requires that files be manually converted to the compressed format. They're uncompressed in parts as requested, but this is only kept in RAM. I'm not aware of any built-in background task that converts files to/from the compressed format.
You can set the "Compressed" flag of a folder and from then on everything in that folder will be compressed/decompressed transparently. I have most of my disk compressed that way and never have seen problems.
That's cool, I hadn't thought of that. But does it identify "hot"/"cold" files that might benefit from being converted automatically to/from the compressed format? That would be a very nice feature to have.