It's a very promising pick. And because of his amazing athleticism and wingspan, RW3 has the chance to be a big man steal much like Rudy Gobert, Clint Capela, Serge Ibaka, Deandre Jordan, etc. etc.
Now will he turn into that, nobody knows for sure. It all depends on how much desire he has to become better and become one of the best in the game. The ceiling is very high, but it all depends on Robert if he's going to become one in a long line of Celtic big men greats. If he can reach the level of Robert Parish I would call this pick an amazing steal.
One word of caution though, big men usually take longer to mature than other positions. So we might not see his true benefits until year 3. Or if we are lucky he takes a big step in year 2 and has everybody talking about him the way that Jaylen Brown made that year 2 leap into relevance.