Yesterday I finally broke down and bought a 1 month subscription to D&D Insider. I think the main reason why I hadn't done so before was because the main computer that I use is my Macbook, and the Character Builder and Monster Builder are only supported for Windows. Sure, I could run Windows on my Mac, but I'm not going to buy an OS that I'm only going to use for two applications. No thank you. I got around it by lugging my desktop from college downstairs (where the internet connection lives) and installing the CB and MB on that. Then I took it back upstairs and voila, I have all the benefits of D&D Insider (thought I still wish I could use everything on my laptop).
I'm still not sure if it would be worth it to have a continuous subscription. The initial investment has the greatest yields, as you get the CB and MB which contain everything from every published source thus far (mechanics-wise). Don't feel like paying for Martial Power just because you plan on playing a Rogue in a one-shot? No problem, the CB has you covered! Still, having continuous access to the Compendium would be nice (searchable online database with all of the info from the CB).
Character Builder/Adventure Tools
So far the only one of the Aventure Tools that's been released is the Monster Builder. It's still a Beta version, but it's very nice. It's essentially a database of all published monsters, and you can change their level with one click (so you don't have to re-calculate everything yourself). You can also customize monsters, and create your own. Definitely worth it! Hopefully I can easily export stat blocks to PDFs or some other format, so that I can at least view them on my Mac outside of the program and print them if need be. I'm really looking forward to the unreleased Adventure Tools, and I hope that the next one is a mapping program.
The Character Builder I've already discussed a little bit. It's pretty nifty, with the main benefit being that you can build characters without having 5 different books sprawled out in front of you. I think what I enjoy even more is the content. I finally have the Assassin class (D&Di exclusive) and all of the options from Divine Power. I especially appreciate finally having the Artificer without having to buy the Eberron Players Guide. It's also really easy to find stuff from Dragon without searching through all of the PDFs. Mac users certainly get screwed by not being able to use these 2 excellent tools.
These alone aren't worth the subscription. I find the layout of Dungeon kind of annoying, with the adventures as presented being difficult to utilize. I actually haven't read through much of anything though; just seems so daunting and the reading so dull. Unless I found a specific adventure that would apply to my campaign, of course.
Dragon is better, though it's fairly well-known that some of the content is broken (i.e. overpowered). It's nice to have a steady stream of published material though. Each month the Dragon material (as well as anything from a newly released book) is added to the Compendium and the Character Builder. This means that if you don't have subscriptions every month, you can't update the CB. Still, updating every few months doesn't seem like such a bad deal, especially since you can keep track of the content offered to see if anything is of interest to you. If they release another D&Di exclusive class, release a crunch-heavy book, or do a really good article on one of your favorite classes, it would be a good idea to get another monthly subscription so you can get the updates. Another nice perk is that debut content appears in Dragon. Right now one build each of the Psion, Monk, and Seeker have been released, all of which will appear next year in the PHB3. I probably won't purchase that books since I've never been a huge fan of Psionics, but the Monk is neat and the Seeker looks very cool, and it will be nice to get these two classes without having to buy a book filled with a bunch of Psionic stuff.
The Verdict: The subscription was worth it, but seeing as I'm currently unemployed it's unlikely that I'll continue it beyond this month. As content is released, I'll almost certainly re-subscribe in the future.