The game is worth it for what it is now for the price it is set at.

The price may even go up when it is complete...

Updates to complete the game will be free.

So it is certainly worth it even as an incomplete game.. even if they stopped doing anything and abandoned the game it would be worth it.

They have no intent to abandon and are still developing the game... if the slow pace and taking years to finish will bother you then don't buy it and just wait.