Taska looks like the love child of Things and The Hit List. Launch the app on either device and you're presented with the basic elements of a GTD app -- inbox, action, next, projects, etc. One thing that Taska does (also something I loved in The Hit List) is a shopping list right on the front page. Once click and I can either see a basic checklist, or further sub-divide it into stores with its own checklists.
Taska on the iPad has a very elegant UI. The front page takes more cues from The Hit List with a row of tabs down the left, while the input area of the app resembles pieces of paper stacked on a table. The same effect is carried over to the iPhone, where checklists resemble a white Post-It note.
You can add new items and set them up either as a task, project or a checklist. It'll go into your inbox, where you can further tweak your task to your heart's desire with context and tags. One really neat feature is a button that allows you to filter out tasks based off specific criteria such as time, tag or context. It's also easy to move items among projects and change the icons associated with different projects, lists, etc.
Another feature, taking its cues from OmniFocus, is the ability to set a context map, showing on Google Maps where your tasks need to be done. That way, while you're out, you can see if you're in the general vicinity of where a task must be completed. The maps aren't visible on the iPad, which makes sense -- you're most likely not going to be having it in your back pocket while running errands. Likewise, setting up the locations can only be done on the iPhone version for now. A context with a location on the iPad had to be reconfigured for the iPhone when I sent it over e-mail.
The hardest part about Taska is the learning curve associated with some of the buttons. You don't quite get what they do at a quick glance. On the iPad version, the sync button is nearly invisible in the bottom left corner. The iPad version could really benefit from the more robust help file that's on its iPhone sibling. In addition, while you can create new tags at the same time you're adding a task, you have to create a new context on its own then add it to your task. Once you've created a context for the first time, you'll have it in the dropdown list for future tasks.
Taska's biggest downfall at this point is the lack of proper sync between its iPhone and iPad versions. Currently, the closest you can get to syncing both devices is linking them both to a Toodledo account, which Taska supports. This is something that will be resolved in a future release, says Tong Lau, BitAlpha's founder. Lau said direct device-to-device syncing will be available as soon as possible.
Until then, you're left with one of two ways to bridge the devices -- either through Toodledo or e-mail. Setting up both accounts with Toodledo was easy, but the results were poor. The project I created on my iPad never reached my iPhone, and the shopping list I made on the iPhone lost all of the subtasks. The best way to get the items to transfer was to just stick them in the inbox of one device and slap a tag on it so it'll land in the inbox of the other device. Toodledo itself hid the tasks that I brought in from Taska and it took reconfiguring some settings to get them to show. Once I did, it worked properly for future syncs.
The other way to get somewhat of a sync is to e-mail yourself the project, list, or task. Tap the e-mail icon and you can e-mail yourself with a link that will automatically place the associated product in Taska. This works beautifully when moving anything that has a sublist from iPad to iPhone, but does not work at all when you go in reverse. I found myself tapping at the iPad link in vain before giving up.
Taska also lacks push notification alerts, which Lau says will be implemented after the release of iPhone OS 4.0. The new OS utilizes a more reliable push notification system.
Overall, at this moment, Taska is a great planner to use on the iPad on its own. The price, US$4.99, makes it a much cheaper alternative to Things. An additional $4.99 will also gain you the iPhone version -- which you either want to use on its own or to carry around various shopping lists sent to the iPhone from the iPad version. The iPhone also has the context map feature, which on its own makes it worth the price.
Once these kinks are worked out, I can see Taska being a very formidable challenger to Things, provided you are the type of person who doesn't need a desktop app.