I have seen this referenced in several questions. I've actually seen people use links into the DDI website in their answers that I can't get to since I don't have a subscription. I know its an online resource for D&D material.
What exactly is it? Is it more than an online rulebook?
DDI consists of several different parts, each of which are imho quite worth the money. If you have a subscription to DDI you get:
Access to the Dungeon and Dragon magazines. While the individual quality may vary I've found both magazines a very good resource for ideas, both as a player and a DM.
Access to the Compendium, a web interface to all published races, classes, items, monsters, and rules. You get the full text, including any errata, without having to haul your books around.
Access to several other tools. These include the Monster Builder, a program for the DM to customize his monsters, as well as some Flash utilities useful to create ability scores and build encounters.
Updates for the Character Builder. This is probably the single most valuable point of all advantages. The CB makes building legal characters a breeze since the program takes care of checking feat prerequisites and other calculations for you.
And last but not least a nifty icon next to your name in the official forums. ;)
I personally find DDI to be a valuable resource although it is definitively not required. It makes many things easier (building characters, customizing monsters, checking a feat or item without having to browse through a dozen books to find it, ...) without invading too much into the game.
One "disadvantage" should be noted, however: new material always appears one month after the book's release date in DDI - after all, WotC needs a motivation for people to still buy their books. ^^
The "Learn More" links on the subscribe page are informative.
Note that some of the features, such as the Character Builder, are Windows-only, so Mac and Linux users are out of luck.
