Really enjoying what I've seen of Galak-Z so far, and it's a damn steal at the PWYW level in the current Humble.
It kind of controls like an amped-up Asteroids in that you control the direction of your vehicle and move around by thrusting. However, you can thrust in any of the four cardinal directions, which allows for circle strafing and firing while retreating. It takes a little getting used to, but it allows for some really quick and fluid movement once you get the hang of things. There's some basic stealth elements, too: enemies have vision cones and you can be heard while you're thrusting (no idea how that works when you're flying around in space, but whatever).
After the first section of the game, you get the ability to transform back and forth from a ship to a mech at will, and each one has a completely different fighting style. What's great is that the transformation happens so fast that it's possible to switch quickly between them in the middle of a fight. If I see a group of tough enemies, I can start out as a fighter and shoot a volley of missles into the group to soften them up, switch over to the mech and grapple one of the larger ships to use as a shield, punch the hell out of it while I'm holding it, throw it back towards the other ships, damaging everybody involved, dash forward with a charged sword swing, to do a good chunk of damage to the cluster of ships that are still trying to get their bearings after being smacked around, switch back to the fighter and circle strafe around whoever's left while blasting them with lasers.
It all feels really fluid, but with enough of a delay between the actions that it doesn't feel overly spammy.
Even better is that the enemies don't seem to have any sort of universal AI. Sometimes a ship will try to run away as soon as you get its shields down, then come back when they're recharged, and sometimes a ship will just charge straight at you with no regard for its own safety. I haven't been able to tell if it's based on if they've got friends around, or the type of ship or anything, but it gives a nice variety to the combat.
The game is broken up into "seasons": 5-level chunks that you have to beat without dying in order to progress. The game has a mode where you can restart a level if you die, but that almost seems like something added as a concession to people complaining about the other mode. I've been playing the "restart a season if you die" mode, and I've been finding it plenty difficult, but not crazily so. Of course, I'm only on the second of four seasons right now, so we'll see how that goes later on.