after trying to use Aptana (Eclipse Helios w/ plugin) for debugging i gave up, it was too slow and cumbersome. netbeans (version 6.9) is much better IMHO, somehow it doesn't get much good press but i find it much faster and easier to use for debugging. (i guess i am not really used to eclipse, but i find the 'perspective' concept not very intuitive) The only downside of netbeans is that it doesn't contain a 'direct' window like eclipse while debugging, a shell where you can run live commands against the program in the debugger.
for code completion, i use a the watir-webdriver gem, which encapsulates the watir elements in other objects, ie netbeans can't figure it out anymore, but i didn't have much hope for the code completion to work in complex situations to begin with.