I've been a Todo user since the iPhone app, then bought the iPad version, but I never tried their online service since Todo ties in nicely with my cloud todo service of choice, Toodledo.
That may change now that Todo for Mac has been announced for the Mac App Store and features cloud sync with the Todo Online service. Previously you had to use Appigo's sync client on your Mac to sync your iOS apps, or use a 3rd party service like Toodledo.
Todo for Mac is a compelling app even
