I only tried it (the free plugin for Eclipse) for two weeks and gave up.
In Aptana’s editor, you can select a piece of code and press Ctrl+Shift+F to format it, which is like the Java editor. That’s good. But the formatting result is not as good, unfortunately. I couldn’t configure it to generate the same formatting as Java code. So actually this good feature is of no use for me. Even worse, when you press } to close a block, it will “help” you format this block. Clever, but ugly.
Aptana shows its suggestions for you almost every time in type in a character. But the suggestions are absolutely useless most of the times.
Features described on its homepage are quite promising but I didn’t try them all. These two mentioned above are already too annoying. So I switched back to JSEclipse.
I use JSEclipse most of the time. It’s very simple compared to Aptana. You can see a list of features on its homepage. For me, the most useful feature is that when the cursor is on some identifier (variable, function name, etc.), all occurrences of the same identifier are highlighted. Even though sometimes it will wrongly mark some extra identifiers, it doesn’t matter much.
Sadly it’s acquired by Adobe. You see, the links given above are adobe’s. The original homepage, however, is here. The last update to JSEclipse was on April 2, 2007. So it’s almost dead. It’s really a pity. See this post in Adobe forums.