The idea behind Habits is excellent and the app is quite simple. You just install and begin to use as there is no learning curve, the app is completely intuitive.
Habits simple approach also helps its own use. To keep a daily control of habits is a habit itself and need to be achieved. An app bloated with features would make harder to develop such a control, but Habits avoids this with its straightforward interface.
The app offers integration with Facebook, an ubiquous feature these days. I consider much more adequate to have a kind of alert option using native OS X notification or Growl integration.
Habits interface could be improved in two aspects. It may do a better room distribution to show more habits in the list. To reduce a bit the size of icons and text would produce this. And the calendars could advance in weekley basis instead of the usual monthly view. Weeks are more relevant to habit development and to jump between months will hide the yes/no marks from the past month.
Another point to improve is the habit entry. You can only use 20 characters to name your habit and there is no way to edit this name.
A welcome feature would be a way to save and restore habits history for cases when the user switch to a new computer or need to do a complete system reinstall.
Habits initial release has a critical problem regarding the font embedded into the app. To design its nice chalkboard UI, developers used a typeface called Pencil Pete trial, a free font with just the basic A-z characters. Hence you can write just in plain English as letters like á, è, ü, ç, ẞ, œ or ñ are not available.
The use of Pencil Pete may also be improper as it is a trial font. Its author says the embedable version is available for sale in his site, what makes uncertain if one is allowed to include it in a commercial product.
With some small improvements and an adequate font, Habits would be one of these great 'small' apps which help you to get a better life quality. By now, these issues seriously limit how good it can be.