Customization of workflows complexity depends on how complex the requirement
for the workflow  is all about. If it is of just sending the document from
one library to another library, we can do it directly MOSS Out of Box or
else SharePoint Designer. If the requirement is too complex and has alot of
mappings for the workflow only then we need to go for Visual Studio.

But for the firs two options it doesn't need to be write any coding and
ofcourse to have more customization and more mappings have to be done for
the workflow, obviously we have to go for Visual Studio 2005

I am currently developing the workflow and I am developing it using MOSS out
of the box as the requirement doesn't have much customization. The following
links have a lot of information about in and out of customization of
SharePoint Workflows.

msdn2.microsoft.com/en-us/library/aa830816.aspx