Incredibly slow most of the time. Also I completed the story mode with on double teagles and auto-guns. I never bought a single gun with limited ammo or used a grenade/repair kit. The only level that gave me trouble was the one with all BB guns and snowball throwers because they can shoot from off-screen. The final level in particular was crazy easy.
Up arrow should be the same as space. There is no reason this couldn't be played with one hand. It feels like a good start. I would prefer more, less powerful pickups along with the usual set of magnet + pickup value upgrades.
Here is the build I used for the final two rounds and beat them easily. Blocks are listed from the bottom up. column 1: sleeper, piercing, digestion, cannon. Column 4: Sticky. Column 5: Sticky. Column 8: sleeper, piercing, digestion, cannon. Had no problems with this build. I pretty much used this format the whole way through the game to get all gold souls. Before advanced blocked like digestion and cannon I just alternated sleeper and piercing. For the levels with ghosts I replaced column 1.3, 4.1 and 8.3 with eye blocks. Plus to keep alive!