In fact I do have Transhuman space, Dragonstar, Bulldogs!, Traveller 20, Star-Wars d20, Blood and Space, and d20 Modern (soon I will get d20 Future). Concerning Star Munchkin, I witnessed it today at my LFGS. Intrigued I got a look... and found it was a d20 game! Since I am collecting d20 sci-fi games (not all of them though, as for example I am not going to buy Babylon 5 , for I fon't like the way they tweaked the d20 rules), I bought that little book!
So, Star Munchkin appears to be a small d20 supplement for playing sci-fi in a comical and cartoony way. I regret that one must also have the Munchkin Player's Handbook (as skills are not described for instance), but I think that gamers used to d20 games and having at least one core rulebook (I suppose that having d20SW would be best in this case) can use it without problems.
As I see it, Star Munchkin is good for an adventuring session or two, to make a break from a regular campaign. I also believe that to really have fun with this one, you need to have a really good DM, and also good players (and probably no novice players).