Unfortunately you are asking for a subjective answer.....
Its already being used in MOSS, its very easy to extend, its easy to extend
further with Visual Studio. I've nopt seen a lot that you need to do that
you cannot with Sharepiint designer. So, how far do you need it to go! It
doesn't have dedicated workflows pre-created for all common tasks, like
timed events, automatic routing etc. If you need these you have to create
them, or use a third party that has them pre-built.
Against what? WWF can handle 100's of thousands of running workflows - each
uses memeory and incurs processor cycles. You have to architect this in
regardless of what workflow solution you are looking at. Products like K2
can offset some of this to other servers, but at a cost..
None of them are easy to change once a workflow is running. Its failry asy
tyo take an exising workflow in WWF and create a new on off it.
That really does depend on what you are trying to do. WWF integrates
seemlessly with forms services, infopath, SPD etc - that may not be the case
with other products - yo0ur cost may be high because of this, or reduced if
its a benefit.
John Timney (MVP)