upgrade from 1.5.2 to 2.0.3

I’ve done it without any problems.

Usual precautions, turn off plugins during upgrade, check them one by one as you turn them back on. And check to see if any of the plugins are on the not compatible list. http://codex.wordpress.org/User:Matt/2.0_Plugin_Compatibility

You also might want to get the 2.0.3 tuneup plugin. It fixes the one issue with 2.0.3, and does a couple of other tidy ups. http://txfx.net/code/wordpress/wordpress-203-tuneup/