Asking what going a cruise is like is similar to asking what staying in an all-inclusive holiday resort is like. No two resorts or cruises are alike. It depends on price, location, expectations, your personality and what you want out of the experience.
I have been on two major-market cruises. The first was to the Caribbean to try it out and a second to the South Pacific on a smaller ship. The second cruise was to challenge my initial impression that cruises are tiresome, somewhat moronic experiences that cater to the unadventurous and dull. The second trip only served to confirm and solidify my first impression.
Your mileage may vary.