First of all, it is very nice to unpack once and have your hotel travel with you, rather then the hectic routine that accompanies the stop and go itineraries of air and land tours. But besides the more relaxed mode of travel, there are other significant advantages. Nile cruises often visit a wider variety of antiquities along the banks of the river. But equally important, they also allow the tourist to gain a perspective of rural Egypt, where people live much the same way they did even thousands of years ago, in mudbrick homes, tending their fields with wooden plows and moving produce via donkey. It is a wonderful experience to sit on a shaded deck of a floating hotel, sipping an iced beverage while watching 5,000 years of culture slowly drift by.
Nile cruises may very considerably, but typical Nile cruises are either three, four or seven nights. The shorter tours usually operate between Luxor and Aswan, while the longer cruises travel further north to Dendera, often offering day tours overland to more remote locations. Therefore, a fairly complete 14 day tour of Egypt might include several days around Cairo, seeing the pyramids, museums and other antiquities, a short flight to Abu Simbel in the very southern part of Egypt surrounding a seven day Nile Cruise.
The usual cruise is aboard a Nile cruiser, often referred to as a floating hotel. Indeed, the better boats have most the accommodations of a land based hotel, including small swimming pools, hot tubs, exercise rooms, nightclubs, good restaurants, stores and even small libraries. Depending on what one is willing to pay, rooms may be very utilitarian and small, 
or larger then some land based hotel rooms. Some boats even have suites available. Better boats will always have private baths, air conditioning, and TVs. It is common for there to be video movies each night, and some boats are equipped with cameras allowing passengers to view the countryside from their TV. Floating hotels also offer various entertainment. Many of the boats have dance areas with disco or even live entertainment, and most offer a variety of nightly shows. These might include cocktail parties, Nubian shows, belly dancers and whirling dervish, plays and even dress up parties where guests don traditional apparel. Like land hotels, meals onboard most Nile cruisers are usually buffet style and include hot and cold food along with a variety of international and local cuisine. Most all boats have good modern water filtration, which is fine for showering, but it is still recommended to drink bottled water, which the boat will have aboard.
A much more adventurous style of Nile cruise, very different from the floating hotels can be arranged aboard feluccas, Egypt's traditional Nile sailboat. Most falucca trips are short, 
enjoyable trips of several hours, but multi-day felucca cruises can be arranged aboard larger vessels traveling between Aswan and Luxor. There is really no comparison between cruising the Nile on a floating hotel and a falucca. The accommodations on a falucca are primitive. Tourist sleep in the open on deck and the sailors double as cooks.
Around the middle of April, locks on the Nile river are closed due to water levels, ultimate time for a Nile cruise is between October and mid April, when the weather is fairly cool, but the locks are all open. However, most cruise boats operate all year. If the locks are closed, cruise operators will arrange boats on either side of the locks, and a transfer must be made between boats.
Finally, pricing, as with land hotels will also have a large range, based on both the boat and the accommodations. Expect decent boats to range in price between about $55.00 USD to almost $300.00 USD per night, with seasonal increases of between 25% to 50% during Christmas and Easter.