For those who may use both it and DOpus, how well do the two integrate? Is TeraCopy worth the purchase (BitsDuJour has a 50% off sale, with a pending major upgrade to TeraCopy), or is the Windows/DOpus copy mechanism/speed sufficient?
It is probably going to depend on your setup whether it is better or worse.
All programs ask Windows to copy the data in similar ways. The speed should be as fast as the hardware will go except for quircks of the hardware, drivers, antivirus etc. So the only way to find out is to try it.
Different buffer sizes can also affect things, and it's worth experimenting with them if Opus isn't copying as fast as you think it should on your hardware.
And remember measuring copy speed is very complex as different tools may copy more or fewer attributes/metadata (a significant cost with lots of files), they may also report being finished when the data is really still being written from the OS buffers, may also report speeds differently, and also are affected by previous tests (which cause data to be cached, or may trigger activity to inspect new files which is still ongoing when the next test is done).
I use the free version of Teracopy and it works well with Dopus. I mostly use Teracopy to do my copy/move operations. I cannot tell if Dopus is faster or not since I never did any testing.