The computer's useful to train against at different levels, for sure! But you'll still want to play against human opponents, too.
Human players are unpredictable at times, and are more likely to try dubious things against you that engines won't (like unexpected sacrifices, sudden pawn storms, et cetera...)—and if you want to really improve, you'll want to get some experience dealing with those unexpected situations, too.
A mixture of both human and computer opponents would be your best bet, I say.
I'm curious to know if anyone has an opinion whether it is more or less useful to play chess against the computer (slowly increasing the level game after game) than playing live players on chess.com? I've started using the computer on chess.com and set it to level 5, planning to go up as I improve (hopefully.)
Thanks for sharing your point of view!