One of the best games on the NES in my opinion...
I just got my copy about 4 months ago and have now beat it various times. The difficulty is very high of course, but once you beat it the first time, it's like riding a bike.
You mean... you actually beat the rat race and clinger winger without cheating with single frame advance
I think I got into it after watching AVGN and Mike play it with Bootsy. I'm proud to say I've beaten this legendary game and if I can do it, you probably can too!
Personally the first part of the game that causes me major problems is the Volkmire Inferno (still beatable) - Intruder Excluder is hard (especially the boss which is random and unpredictable) but still beatable, however the 3rd Rat Race is really insane. I'm not even sure if I beat it or not but if I did I was running out of lives and got beaten by the boss. I think that happened but I'm not even sure right now. At least what I am sure is that I never reached the clinger winger legimately.
I beat the game a lot using save states and frame advance, of course, which also helps to beat it legitimately, since I can get infinite retries on hard sections and train myself that way.