I meant to write up a bigger and more comprehensive post about this, but since there's a time-sensitive aspect, I'll just do a quick discussion for now.
The time-sensitive aspect is that there's a $10-off discount coupon code for Things that expires today. So if you want to buy Things at the early-adopter discount price, then you'll have to do it today. The coupon code is THINGSPRESALE20; according to various posters on the forum, it's fine for anyone to use that code.
Unfortunately, even though Things has been in public beta for a year and has just gone 1.0, it still has some flaws and bugs and missing features. So if you're interested in trying it, download the free trial edition and poke at it for a bit before deciding. Sorry I didn't suggest this days ago, when there was still time before the coupon expiration.
Most of the to-do-list applications these days adopt, to one degree or another, the Getting Things Done philosophy/approach to task management. Apps vary as to how tightly or loosely bound to GTD they are.
I'm personally kinda dubious about GTD, partly because I have too many to-do items to ever get all of them done, partly because it requires discipline of a sort I can't muster, partly because of the fanatical fervor and glowing eyes that GTD disciples evince when the topic comes up. However, I do like the idea of having an organized and hierarchical to-do list.
There are about four serious to-do-list apps for the Mac. Most of the reviews I've seen have dismissed one of them as too hard to learn and having too clunky a GUI, so I haven't tried that one and won't be mentioning it. There are also at least three or four other lightweight to-do-list apps that don't try to implement GTD; those may well be sufficient for many people's needs.
In this entry I'm only going to mention what most reviews seem to consider the top three Mac apps in this space: iGTD, OmniFocus, and Things.
I started using iGTD a while back. I like a lot of things about it, and am pleased that it doesn't rigorously enforce GTD, but there are various aspects of the GUI that never really made sense to me, and the documentation is minimal at best. (There's at least one common/prominent icon for which, as far as I can tell, there's no explanation anywhere on the website or in the app.) Somehow I slipped out of the habit of using it--perhaps just because it felt like more overhead than I wanted, perhaps because I felt guilty about the number of tasks that I wasn't getting done.
I considered going back to it recently, so I stopped by the website to find out about progress on version 2, which was released as a public alpha quite a while back. (And to see whether the developer was working on an iPhone version.) But there didn't seem to have been any progress lately. I poked around in the forum (which appears to be completely broken today, dunno what's up with that), and saw posts suggesting that the developer of the app may be too busy with other stuff in his life to keep developing it.
So I started looking at other options.
OmniFocus seems to be widely considered the top-tier Mac task-management app. It's a mature and powerful application, with lots of features. It has an iPhone version that syncs with the desktop version.
But my impression from most of the reviews I've read is that it's a hardcore GTD app, designed to appeal to people who are heavily into GTD. And when I watched the demo video, several things felt weirdly complicated to me; I don't think the underlying philosophy of the app fits the way I think.
I may also be projecting from my limited experience with other Omni apps, which I've felt the same way about. My impression of those apps tends to be that they're powerful, pretty, and designed by aliens. But lots and lots of people love them and swear by them; it's clearly a personal-taste thing. For example, Kam is a very happy OmniFocus user.
OmniFocus costs $80 for the desktop version, and another $20 for the iPhone version. Those prices both seem expensive to me, based on nothing but gut feeling of the kinds of apps I expect to get for a given price.
The third major option is Things. Things is a new app, created by a small team; it just launched its 1.0 version about a week ago. Almost all the reviews say the same thing: gorgeous Mac-feel interface, lighter-weight than OmniFocus, not as tied to GTD.
Things costs $50 for the desktop version ($40 with the discount code mentioned above), and $10 for the iPhone version.
I've been using the desktop version for a week or so, and I just bought it last night. I'm still not sure it's everything I want it to be. It doesn't let you nest projects, for example--you can put tasks in projects, and you can put tasks and projects in "areas of responsibility" (a slippery concept that I'm only beginning to get a grasp on), but you can't put projects into other projects, or create sub-tasks. The developers say they're working on a way to make sub-task desirers happy while still not allowing huge hierarchies that obscure tasks and make it hard to get stuff done, but who knows when that'll be available.
In the meantime, most of the stuff that other task management apps handle with various strictly defined concepts (like GTD's "contexts") is handled in Things using tags. Just like del.icio.us tags, or blog-entry tags, or Flickr tags, or Gmail tags. It has the same advantages and disadvantages of those other tagging systems, though it does allow hierarchical nested tags (which is a huge plus for me over other tagging systems). A lot of how useful this feature is depends on how well you design your own personal tagset.
There are a few minor GUI weirdnesses and non-intuitive bits; some of them are bugs, but most of them are just the way Things works, though some of those will be improved in a future release. If you run into problems, I highly recommend reading the FAQ (and skimming the forum) before filing a bug.
There are also some outright bugs. The only serious one I've run into so far is an intermittent failure to sync between desktop and iPhone versions. I'm guessing that'll be fixed soon.
Anyway, this entry isn't meant to be a full detailed review. My overall impression of Things is positive; it feels to me like it's at approximately the Goldilocks "Just Right" balance (for me) of complexity with power. But a lot will depend, for me, on whether the development team can deliver on missing features and fix various bugs.