DH grew up cruising. He likes it because you unpack your bags once, don't need to check into different hotels to see different places. We've gone on three.
You need to go in knowing it's not really all inclusive on the major lines. Gratuities are virtually required (technically you can stiff the people who worked for you but that's really low in my opinion.) That's part of the cost to me. Optional items are: drinks, excursions, fancier food, services (spa or quiet adult areas), gambling, photos, internet, etc.
Like any good mustachian, we will deal hunt, which often means getting some onboard credit offered to us for the same cruise price offered elsewhere. We also learned that if you own Carnival stock, any Carnival or subsidiary (Princess, which is what we did), will give you onboard credit per stateroom for being a shareholder. (Our stock has gone up nicely since buying it otherwise we wouldn't keep it.) Finally, if you put down a deposit of $100 and let them hold your money (fully refundable if you don't cruise in 2 years), then you get more on-board money. For about a 50% return (varies by stateroom type and length of cruise), sure you can hold a tiny bit of my money.
With this money (I think we had $300), we were able to pay for our gratuities, buy pretty much all the drinks we wanted and have a fancy dinner at a specialty restaurant. We do excursions with local tours to get a better experience, better price, and more money to locals. We learned from reading cruise ship boards how to get free/discounted drinks (returning members attending a presentation get a card for a drink, at the happy hour you can buy two identical items for the price of one. This included buying beers unopened for the next afternoon, and buying two half bottles of champagne to use over 4 breakfasts we brought back to eat on our balcony (mixed with the orange juice only offered on this line at breakfast) to make mimosas.