I finished it. The game is great so far.
As a comparison to Tim's previous games, I liked it more than Day of the Tentacle, though not as much as Grim Fandango. The latter is really hard to match/top, though.
Really looking forward to part 2!
It is a shame the first half is Steam exclusive. I would be quite upset about it if we weren't promised the final DRM free build once the full game is out. That said, I really do think they should offer an alternative to those who would prefer not to use Steam. I certainly would have taken it myself if I'd had the option.