I would say that I'm OK.
When I first got Civ IV I played the first 2 or 3 games on Warlord. Then I switched to Noble and played on that for a while before moving up to Prince. My gameplay is now exclusively on Prince, and I have absolutely zero desire to increase or decrease the difficulty level. I would say that I win somewhere in the range of 80% of my games.
Like others have said, Civ is so customizable, even in the general settings that can be changed, that one of the most important aspects of this game is knowing what you will enjoy and choosing the settings that will make that game happen.
The thing I've found is that to really succeed in Civ you need to be decisive, take risks and do everything to the extreme. For example, if you wish to warmonger, then you need to have a very legitimate need to have a building before sacrificing hammers on construction instead of military units. The opposite is true in regards to Culture wins. Taking the middle road (compromising) is generally punished by the computer.
This is the main reason I prefer to play on Prince. I hate to micromanage, and I would characterize my playstyle as aggressive warmonger-builder. I like to win, but I like a good fight. And I love to have a strong military, but I am able to immerse myself in the game more if I also have the ability to build some of the wonders and buildings I think my people would like were my empire to be real. I play for fun and it requires too much concentration and micromanagement for me to compete at higher difficulty levels.
Only a few small changes have allowed me to move up in levels. The biggest and most important change I've made is to be decisive in what I plan to do at the outset of the game, and to dedicate myself to this goal throughout. For example, I usually have random leaders, so as soon as I see my leader and my immediate surroundings, I try to generally think of what victory I would like to pursue. In my experience, I have the next couple dozen turns to solidify my victory choice before it becomes increasingly difficult to achieve any victory.
Unlike the really good players, though, I don't fully sell out for my chosen victory condition or micromanage to make it happen faster; rather, I try to play with that general victory in mind. The more discerning I am with my choices, especially in the early game, the better I do. The wide majority of my losses come in games where I am forced to alter my Grand Strategy any time after the first 50 turns.
Sometimes I am utterly terrible at this game, and sometimes I think I display flashes of brilliance. In any case, I agree with the other posters - It's a game, have fun with it. I do, and that's why I've played Civ so long.