Without a doubt, it's cheating.
A game is developed in a certain way, the way the developers intended for you to play the game. If they decided they'd make it the hardest thing on Earth, then that's how you as the gamer have to conquer it. Use whatever skill you have to figure out a way to beat the game. For example, in the case of Space Ace, when I was younger, that was one of very few games that I owned. It was difficult, but I was able to master it with some patience. I can now run through the game with no problems (as sad as that sounds).
Cheats built into a game are a different story. The developers have said "Yes. We know the game is difficult. Here's some help". Yes, that's cheating, but it's cheating with permission...sorta.
When it comes to save states however, you're pretty much breaking the rules that the player has set out. It's like playing Monopoly and you end up going to Jail. You can't say "Gee. That sucked. I'll just go back to my last position and I'll roll again". Those aren't the rules.
I agree, save states are EXTREMELY handy and they make some games much more playable, but they're still cheating. I use them frequently, but only when I don't have enough time to play a game through. The way I use them is as a one-off restart point. For example, if I were playing The Last Ninja 2 and I made it to the 2nd level but I had to stop playing for some reason, I'd save my state. Later I'd come back and restart from that point, however I'd only use it once so it's like I never left at all