Recs.
Updated
Specs
Pros
Pro Dependently typed
The type system is even more powerful than Haskell's, which is more powerful than Scala's which is more powerful than Java/C#. What would be runtime assertions in a language like Java can be built into the type itself in Idris. If the code actually compiles, it probably does what you want. Most programming time is spent debugging and writing tests. You don't have to do that so much in Idris, so it makes you more productive.
Pro Domain driven design and type driven development
Because of full dependent types in Idris, the programmer can focus more on modelling the domain with types and waste less time fixing common bugs that the type checker will catch. Dependent types help apply type driven development and a lot of code auto generation, making the compiler and type checker an ally in developing working software instead of just getting in the way.
Cons
Con Different semantics from Haskell
Idris, while similar to Haskell, has strict semantics, which may cause some confusion if your backend is done in Haskell. If using Idris, it would make sense to do the backend in Idris as well, if not for the fact that Idris currently has fewer libraries available for web development than Haskell.