Very good! My own entry, [InvadTris](https://ldjam.com/events/ludum-dare/41/invadtris/) took a similar approach, you should try it out and let me know what you think.
I thought playing Tetris upside down made the game really challenging and different. The concept is great, and what you have here is already very good. But to make it even better, I have a few suggestions:
The game needs sound. Some good BGM and laser shooty noises and tetris block movement and landing noises would enhance this game a lot. It's not complete without audio.
I'd liked to have seen more interaction between the shooter enemies and the falling block. They can't hurt you, so all they do is give you opportunities for bonus points. They are pretty safe to ignore, and have no real impact on the game if you don't care to shoot them. It's also not worth while to try to move out of your way to shoot them. Mostly I just focused on placing the blocks and completing lines, and held down the fire key and got bonus points incidentally when I did.
Due to the speed at which the block and the space ships passed each other, it did not seem possible to destroy enemy ships from the sides, so that aspect of the game could stand to be tweaked. Maybe give the ships more vulnerability from the side, or I don't know.
Some power-ups to improve your shot strength, or temporary shot boosts that could break tetris blocks would be cool. Some more variety in the space enemies, and even a boss would be great. If the boss ship came down in behind the tetris stack and used it as a defensive screen, or started grabbing blocks and lobbing them at the player, that would be very cool.
The game should also have a Next window so you can see what's going to fall next. And starting the Next block in a random horizontal offset instead of always in the center of the well is an interesting choice, but I'm not sure that I liked it. It's important when playing tetris to have the initial conditions be predictable so you can play your strategy accordingly.
All in all there's a lot that could be done with this game, and I think it would be fun to see it developed further. I hope you do.