I have kept several Nile monitors. They are beautiful animals, impressive and powerful, and also quit ehardy in captivity. However, I have come to the unfortunate conclusion that they are the second least suited of any monitor to be kept as a pet (with only the Komodo monitor being less suitable, for obvious reasons). The problem is that Niles are paranoid by nature. They almost invariably see your presence as a prelude to a predatory attack, and react appropriately - generally by running and hiding. If flight is impossible, however, they defend themselves with teeth, claws, blows from their powerful tail, and deficating all over you. What this means is that captive Niles can almost never be held without either armor or blood, and that they are difficult to even admire since they tend to dash for cover as soon as they see you.
Now all of this wouldn't be so bad if Niles were not so big. But they are. Being big means they need a huge enclosure. They need lots of food. When they relieve themselves, they make a huge, very stinky mess. They need a basking area big enough to heat themselves and provide enough power to heat their cage. This means that Niles will take up an exorbinant amount of space and time - your precious time. It also means that, contrary to the sticker price, Niles are not cheap. Figure you will need to put down around $500 to $1000 for the adult enclosure, and that is if you build it yourself. This includes the cost of the wiring and heat lamps and water dish, but neglects details like pumps and filters to keep the water clean, so you will need to haul around a huge tub filled with sloshing water and monitor fecal material about once a day to maintain hygenic conditions in the enclosure. It also does not include the cost of maintenance. In upstate New York, I was getting $1000 two-month electric bills over the winter for my two large monitor enclosures. Electricity may be cheaper for you (especially if you are on the West Coast), but it will not be cheap!
If you are really interested in a monitor, I would suggest looking into the dwarf monitors like ackies, storrs, or black heads. If you really want a big monitor, check out blackthroats, arguses, sands, and dumeril's. The initial purchase price will be considerably higher, but you will probably get more enjoyment out of your pet and the full price including housing, food and electricity will not be that much more for the large monitors while in the case of the dwarf monitors, you will ultimately end up paying a lot less.
Having said all that, I will admit to once owning a truely delightful Nile monitor. She was skittish, like most of her tribe, but more trusting than most and she really captured my heart. I never found another Nile like her, though.
I have a web page devoted to Nile monitors here:
http://www.pizards.com/hbd/nile.html
Their basic care is very similar to that of argus monitors, which I cover here:
http://www.pizards.com/hbd/tricks.html
Finally, a web page about my "nice" Nile monitor cleo, demonstrating the problems that even a docile large monitor lizard can cause
http://www.pizards.com/hbd/cleo.html
Luke