I just tried updating from 1.5.18 to 1.5.21. When I get to step 3 I received the following message:
Joomla! Update Manager - Step 3
* Unable to read archive (gz)
Warning: gzopen(/home/.../public_html/tmp/Joomla_1.5.18_to_1.5.21-Stable-Patch_
Package.tar.gz) [function.gzopen]: failed to open stream: No such file or directory in /home/.../public_html/administrator/components/com_jupdateman/libraries/joomla/filesystem/stream.php on line 156
Failed to extract archive!