Bubble Tanks 3 is the game that the recently reviewed Dead Metal's engine comes from.
Whereas Dead Metal involves being trapped in a single area with hordes of enemies, in Bubble Tanks you roam from bubble to bubble, taking out various enemies and collecting bubbles to upgrade your bubble machine. The above tank is your basic one; the below has been upgraded several times.
This tank might look pretty big, but the interesting thing is that it's possible to get a tank so large and powerful that 1) you can barely see what's happening, and 2) it takes out most enemies by itself while you're trying to figure out what's going on. I've only done so once because of odd problems my computer's been having with saving data on game sites (I've lost the data for Incursion, for instance, about three or four times now).
So my (brief) conclusion is that it's entertaining to play because you can get tremendous unstoppable juggerdreadnaughts of mass destruction to cruise around in, and that's always fun for a little while. My simple tip is to stick to the central areas for a bit, as you'll need to gather up resources before fighting any serious enemies.
(Incidentally, the game seems to spontaneously generate what enemies you fight as you explore, with the only fixed aspect being the rough strength level of said enemies. Which is also neat. You can also apparently design tanks for yourself and enemies, but this is partly limited by needing a membership with the site.)