Okay, I've been using it for a few hours now. Overall, I love it.
Pros:
* Love the overall idea.
* Really like how it floats at the bottom, reminding me what I'm supposed to be doing.
* I like the funny way it congratulates you on completing a task
* I kind of like how the tasks are laid out, though there are issues (see below)
Cons:
* Didn't work for the first hour ("You appear to be offline").
* UI was confusing at first. You can't add tasks to today, and you can't add tasks to "All". Or, more accurately, you can only add one task. Then when you press "Enter", nothing happens. There's no feedback letting me know what I'm supposed to do, so I had to click around all confused to figure out how to build a task list. Turns out you have to build a list on the left, then star things to add it to the middle. This took a while to figure out.
Anyway - overall, pretty happy with it. I'll likely still use Asana as my master task storage, but move it over to Slash as I work on them. I could see myself moving over to Slash completely, but I still don't know how reliable the storage system on Slash is yet. But love the idea overall.