My advice to most people in most cases is to avoid registering domain names of the form "i"/"e"/"123"/"u" + another word. There are some great domain names that follow this pattern already in use by businesses, as we all know; but the idea tends to be overused by domainers. Ultimately it's a fad, and the gimmick will make more domainers poor than it makes rich.
Some pointers:
(1) Make sure the combination of letter + word makes sense as a business idea. "i" should personalize the product after it--meaning that the product/service/website allows you to do something yourself at home that would normally be done elsewhere. Example: the iBank software, which is for personal banking.
(2) "e" should stand for "electronic". "eBook" makes sense because there are electronic books. "eRefrigerator" is pointless.
(3) "u" should stand for "you"--meaning that the business makes the customer their product. I'll use one of my own as a good example: uPublicity.com, which works because publicity and marketing are all about "YOU".
(4) Never ever ever buy domains like these in any TLD other than .com. Buying them at all is risky. You're going out on a limb by expecting some startup company will think of your domain name and want to pay top dollar for it as their brand identity. They definitely don't want eMetaphysician.info. The only exception is if you're jumping on a bandwagon that somebody in big industry is already pulling--for example, "eBook". eBooks are now real products, and there is no trademark that I'm aware of. So buying eBook.net actually does make sense.
