I'll repost my impression from a gamefaqs thread.
I just beat the game, it took me around 7 hours to beat it. Last boss fight was extremely cool, very Character Action like, I really enjoyed it. Though the game ended sort of abruptly, it's a pity. In the last fight Ivan sort of "transforms" as a character and I was hoping that there're be some more game where we can play as this new Ivan, but BAM and credits started to roll.
Otherwise, I REALLY liked the game. It's very basic in it's mechanics, really barebone, but thanks to that it has an awesome feel of a really old school action/shooter, you just keep running forward shooting the s*** out of everything and then occasionally you face a really nicely designed boss. There aren't that many bosses (8 if my count is correct), but at least 5 fights are really memorable and challenging and I enjoyed them. It's a really simple game, but it has a really nice feel to it, it you enjoy good action games, chances are you'll like Devil's third.
As far as I can tell, there are no unlockables after beating the game BUT in each of 9 missions you can collect 6 items (crap like bottles of sake or magazines or stickers) and I didn't manage to collect all 6 in any of the mission, so maybe they unlock something, don't know for sure.
Also, after a game you get a really touching message from Itagaki thanking the players, really made me feel for the guy, I sure hope he'll be able to find funding for another game, I'd like to see where he goes after Devil's Third.