Pretty Good
I have to say, I really like the addition of a weapon selection because it gives you options for how you want to tackle the boss. However, the aiming system makes it really aggravating, even on automatic, and that destroys the fun of that progress in the later levels. It's very difficult to track projectiles that are seeking you when your movement affects aiming. I think this is one area where the original game is far superior, because you could control the spread and angle of your fire on the fly. Here it becomes very difficult when you're maneuvering to avoid projectiles, but you want to move a different way in order to make a shot. I suppose you can consider it an aspect of gameplay (making the choice, should I attack now or keep avoiding?), but at least give more angles of attack. It seems like we can only aim in 8 specific directions, and there are no half steps, which makes it hard when you're trapped in an area that cannot make a heart shot. I don't know if you chose to remove that aiming system deliberately or were forced, but I think that the original aiming system should have definitely stayed. Additionally, it would be nice if you got more of a cue when you respawn, because sometimes it's hard to track where you've ended up when you're under the boss or when there's just so many objects littering the screen.
Either way, it's still a lot of fun.