Apache WSIF originated as an IBM alphaworks project that was later turned into an open source project. Oracle currently still uses it in its SOA products. The age of the Apache release suggests that the WSIF/big SOA vision never really gained momentum with the open source community. However Oracle's interest in WSIF is maintained as Oracle is a "big SOA vendor". So investing in WSIF at this point probably only makes sense if you plan to use Oracle's products to realize your SOA vision.
However if most of your SOA consists of data services then WSIF may be of limited value as it is based on WSDL and alternate web service styles may require alternate/newer description formats (like WADL).