I really like the idea of this single character who gets powered up by all the collective players. I think the idea could have been pushed more though, such as having individual upgrades giving tiny amounts of power (for example only getting 1 additional hp) but as a collective all us players would be making a difference. It seemed that by the time I started playing all the upgrades had all been purchased, though this might have been a bug? I also got stuck on some level geometry, so a quick respawn method would be handy.
I also want to mention that I found the automatic gun firing to be annoying, the repeated bangs that can't be turned off mean I can't enjoy any of the other sound design. I would much prefer manual.
It's a cool concept, but I feel that its only the tip of the iceberg.