I mainly play on my own. I'd say it is still somewhat enjoyable. With no Big story mode like the Subspace Emissary like the last one, you are restricted to pretty much just battles. However, the challenges add a little and unlocking everything will take a little time.
I mainly play through the classic and All star mode with each character, and a lot of 3 stock matches against level 8-9 CPU's and still get enjoyment out of it.