I've been working on custom levels for Banjo-Kazooie since the level editor was released back in May. Things got kind of quiet on RWC so I put up my first and unfinished map for download. I haven't touched the editor in the while, but I'd like to keep working on it in the future.
If you want to try it out, check out this topic:
http://www.rarewarecentral.com/forums/showthread.php?p=147216#post147216
You have to apply an xDelta patch to your BK ROM and all the information can be found here:
http://www.rarewarecentral.com/forums/forumdisplay.php?f=201