I have tried using Haskell to various smaller projects, such as wishsys and a game that I never got really far into making. But learning a new programming language through the means of hobby projects only work as long as the project is contained and small. For my part, most hobby projects start out with great ideas and grand designs, but end up as a mess since I am unfamiliar with the programming language.
When using a new programming language, time is spent learning the language rather than developing the project. This in turn means that I end up learning the bare minimum to get the job done. And this defeats the purpose of using a project to learn a new language. If the goal is to finish the project, you should have used something you know well and feel most productive with. If the goals is to learn a programming language, you should start out with a small project instead.
For me, project euler is a great way to learn Haskell, because it contains a lot of problems that Haskell (and functional languages in general) is the perfect tool for solving. The projects I mentioned above involves using databases, multiple threads and other scary real world stuff, but I just wanted to learn Haskell. And better yet, once you have solved a problem, chances are you can find someone with an even more elegant solution written in the same programming language you are using. A great way to learn!