Same, here; I have 5h45m of playtime, according to GOG Galaxy, but I never managed to get to level 6 (heck, in a "usual" good run I only manage to get to level 3-2!); I unlocked Melting (well... obviously, since the only requirement is for the player to die for the first time), Eyes, Plant and Horror -- and a cool skin for Eyes.
Like you said, I'm having a blast with the game, one of the best, funniest games I've played in a while (and a breath of fresh air on GOG, with all the strategy and turn-based crap they've been releasing for months), but I'm definitely not good enough at it. Nuclear Throne is really tough, for me, but I'll keep going at it either way, just because it feels so good. :P