The enumerator and iteratee libraries are cool. The reason why I am switching to iteratee is zip. If f and g are two complex functions that turn multi-gigabyte files into final data structures, then Iteratee.zip f g will produce the two data structures in one pass.
And ><> can combine enumeratees.
Unfortunetaly, the library requires some time to learn it. Both enumerator and IterIO are (much) simpler to grok. BiobaseInfernal provides some examples how to use iteratee. Check out Taxonomy.Import for a use of zip.
If there are requests, it would be possible to feature both libraries for importing / exporting of stuff. Iteratee.zip is, however, a strong incentive, for at least one library.