I've been leery of 4e, and the DDI actually makes me a bit more sympathetic to it.
...and it is.
The huge selling point for me is the Rules Database.
The other stuff is good. I'm going to enjoy playing a pickup game on a lazy Saturday afternoon. If this can be paired with a voicechat system, all the better (PLEASE!). The character modeler is so-so, it's not a big thing for me. The Dungeon Builder is likewise. My campaigns are ecclectic enough that both are only useful about half the time anyway (you tell me how you'd design the combat on a wall made of writhing human bodies where the PC's, which include a man in bandages, a hideously obese cannibal, a man wrapped completely in chains, and a nudist, are attacked by skeletal bats from hell with that system, and I'll bake you some brownies).
But I see that the Rules Database will have all the rules....for the books you don't own....
....and I think....wait a minute....I won't need to buy the books!
I spent probably about $30/month on D&D3e, because I'm a hideous collector of things. Every month, almost, had a new book that I'd want to buy. Even if I wasn't buying a WotC product, I'd be buying SOMETHING.
I mean, I might choose to buy the books. If they're awesome, if the artwork is stellar, if I'm being Little Mister Obsessive about it, sure, I'll buy the books.
But the fact that I could still make a character with the newest feat from the newest splatbook, the fact that I could still use the monster from MMV...
....and that I'd be paying only $10 for it that month...
.....that's persuasive. That's telling me I'll actually be saving money with DDI. That's basically Wizards saying "Look, man, we know you don't always want to shell out full price for a book that might not be worth it, so here, have the rules at a discount. $10. If you like 'em, you can get the full thing.
That's totally fetch. That's AWESOME.
T-Rex With Lazer Eyes Awesome.