If you ever get the chance, go. Trust me on this. In my experience, you're pretty much treated like a Persian satrap the entire time, the accommodations are comfortable (if not huge in physical size), and the food and ambience in most cases is like a 4-star restaurant. You can stuff yourself like a paté goose on these things if you want to, and its usually all included in the price of the ticket. They get the "worst" part of the whole experience (the government-mandated lifeboat drill) out of the way right off the bat, so everything from there is uphill. And believe me, "uphill" ain't no cabin up a holler :)
My "home" ship is the Crystal Serenity (luxurious accommodations, they wait on you hand and foot, and they keep the obnoxious kiddage to a minimum), but bookings on Carnival are usually cheaper and the accommodations aboard are every bit as nice. Disney's ships have good deals from time to time as well. Every ship is a little bit different - you can research them yourself by reading the reviews on web sites like cruisecritic.com and cruiseopinion.com.
My stock advice to first-time cruisers is to take twice the money and half the clothing you think you'll need. I have a specific list of things I take with me (such as bungee cords believe it or not) that make the experience perfect for me. Also, book a cabin with a balcony if one is available. There's nothing quite like taking your morning beverage of choice out on your own private balcony, sitting back, and watching the sea go by.
Doggonit, now you've got me looking forward to MY next cruise. I wish I could go on the upcoming FreeRepublic cruise, but I already have one booked for later in the year.