Personally, I update ~weekly all library/apps for months with extra-devel enabled and I have not a single problem with them for now.
But if you want to do this, make sure that you:
-have read http://wiki.maemo.org/Extras-devel
-make regular backups (with the backup app + copy of the MyDocs/memory card partition) and copy them to another hardware storage than your N900
-are ready to do a full reflash and loose time if something go wrong.