I own Killer7 and am going to play it soon. NMH is either a love it or hate it thing. I can't even explain it, really. The game has some glaring flaws that I didn't even mind looking past just to continue with the story and boss fights. The funny thing is that it's supposed to be satire.
I can always just pirate it on PC and have all the DLC for free =)
No More Heroes is a great game. Some people don't appreciate it but if you've liked anything Suda51 has been a part of you'll love it.
MadWorld was a game with great humor, awesome music and entertaining style / play. Overall it fell a bit short of my expectations and I bought it day 1 for full price. These days you can pick it and NMH up for like $15. I would have gladly paid full price for NMH but I actually bought it second hand about a year after it came out.