When it comes to move a course from WebCT to Moodle I have seen three resources people want to move easily: content and its TOC, questions and quizzes. Other tools such as forums, chats tend not to be migrated as generally a forums starts from the scratch each semester.
I will depend on what you want to transfer. We have been migrating from WebCT to Moodle where content is being transfered using IMS CP. We run WebCT CE 4.1 and Moodle 1.6, and content is simply migrated from WebCT with an export function, and it is imported correctly into Moodle by using IMS Resource type. We tried Moodle 1.5.x IMS and things were also working ok.
If you need to move questions, you can use the export questions feature from WebCT and the import questions in WebCT format from Moodle.
My WebCT importer works on data extracted from WebCT CE 4.0 and 4.1. It's untested on other versions (e.g. Vista or CE 6) and unlikely to work with them. However, for CE4, it works pretty well, copying across the vast majority of different item types.
My WebCT importer has just been updated to Moodle 1.6 though, and I'm afraid that due to some database changes the download bundle is for Moodle 1.6 only. If you want a 1.5.3-compatible version you can download an "old version" from CVS as follows:
[just hit enter when asked for a password]
cvs -z3 -d:pserver:firstname.lastname@example.org:/cvsroot/moodle co -D 20060616 contrib/webctimport
By the way, there's a documentation page about WebCT importing which may help.