How far would you go to win a game? This is the excellent question that Spicerack asks over on 40k War Zone.
This is an easy one for me. Cheating is too far. Everything else with in the rules is fair game on one condition - that both players are truly playing to win based on the rules.
I enjoy 40k most when I'm trying to win, so I like competitive play. Sure, I enjoy those epic moments when something cinematic happens and I will never forget my first tournament playing with a fully painted army. But nothing beats crafting a list and pushing as hard as you can to win (this includes pre-game prep too). The winning bit doesn't actually matter to me as much, and placing in a tournament is a measure of my development as a competitive gamer. For me the fun bit is trying as hard as I can to see how good I can get.
There is a fantastic book by Sirlin called 'Playing to Win' (free on his own website). One of the fundamental points of the book asks players to look at what holds us back as gamers from playing by the game's rules, contrasting this against players who use a self imposed set of rules (e.g. I only play with X army, that would never happen in the fluff, claiming that something's not fair because they didn't make the effort to learn the rules properly, etc.) and then judging others for not following their definition of winning.
For me it's really important to agree what kind/style of game I'm going to play with my opponent before the game starts. If it's a relaxed, low-powered game than that's cool (although I still enjoy hardcore more!). If it's an experimental 'I don't know if this is good or terrible but I want to give it a go' game then great. My favourite is still a 'no-holds-barred no-mercy given none asked hardcore' game. I'll add here that all of these can be done in a polite and non-confrontational way!
I will play anything really (still a strong preference for hardcore), but I want to avoid a situation where players do not agree on what 'winning' is, resulting is neither player having a good time.
Competitive tournaments should be an environment where everyone has a common definition of winning (as defined by the basic game rules or tournament variations). In this situation we should be applauding anyone who earns their win fair and square. And then keep on trying to beat them next time!
So I guess what I'm trying to say is that nobody really wins if you're not playing the same game. So check first!