I originally subscribed for the Compendium. The glossary is incredibly useful, and even though I now have a physical Rules Compendium, I rarely reference it when I'm the DM. The online Compendium is even better than a searchable PDF. It also means I rarely buy player splatbooks that are mostly crunch. I tend to stick to DM stuff: the DMGs, the MMs/MV, the Shadowfell box set. AV2 for vehicle and alchemy rules.
There was also a long period where I at least skimmed most of the Dragon and Dungeon content. That's fallen by the wayside a little bit, but there's still a fair amount of content that interests me, particularly fluff stuff like the history of Nerath and Bael Turath that I can adapt for my own games. I've been reading more lately, and it's great to be able to pillage the archives for whatever tickles my fancy. The only thing I'd like to see there is better search capability.
Now I'm also a heavy Character Builder user. It's a lot easier to make a quick character as an experiment to see if the concept is even workable using the CB, and when getting ready to play in a new game I generally make 2-3 characters and then pick one. The CB is great for this. In the game my girlfriend DMs, I also maintain the characters for the entire party, since the other players aren't D&DI subscribers.
Looking ahead, I've tried the VTT, and can see myself using it in the future if it becomes more flexible -- a lot more flexibility is needed on the mapping front. The voice chat is also buggy.
The Monster Builder will hopefully be nice one day, but for now I'm sticking with Power2ool for my monster adaptation needs. I also like Power2ool for building custom monsters and items. I'll probably still use Power2ool for live games even after WotC's suite of stuff is complete, simply because Power2ool has a fantastic interface for use at the table.
So right now, I'm subscribed for the Compendium and CB.