Query is the query language of the O2 object-oriented database system. ICAD Inc market a CAD system for mechanical and aeronautical engineers. The language in which the engineers describe their design is functional, and it uses lazy evaluation extensively to avoid recomputing parts of the design which are not currently visible on the screen. This results in substantial performance improvements. Some other examples of Haskell in practice are found at this web site.
Functional programming does require a change in perspective, which some programmers find hard.
They used to be, but the compilers have now caught up. Haskell programs run fast enough for all but the most performance-demanding applications. Haskell has been successfully integrated into existing applications in a number of ways. These tools have been used to build a number of successful, mixed language systems. Many software libraries have been developed for Haskell.