i did an upgrade this morning for a site i'm not monetizing with virtually no traffic from joomla v1.5.2 to v1.5.4 via Cpanel > Fantastico. was a little alarmed at first at how many things reverted to default but here are things i had to redo to get things back to normal. note, the more manual adjustments you made, the more you'll have to redo. all my plugins still seem to work. just noticed v1.5.5 is now out. edited index.php for template and readded google analytics code edited configuration.php for live site entry. in global configuration, re-enabled SEO friendly urls and reset time zone. had to redo some main menus but i think everything is working
Another thing you might want to consider before you upgrade is your meta tags! If you have applied meta tags and a meta description in your site configuration, you might want to save these in a safe place before you upgrade! I have notice this goes back to the default Joomla Meta Tags, which can be very frustrating! Cheers
It's a bad idea to Update just because a new version is available. When a site is working properly, you just leave it be.