I see this has been brought up before, but I thought I would start a new topic, as the rest seem to be archived.
I have been using Aptana as my main IDE for a little while, but since its most recent update, it seems to work brilliantly as a Silverstripe IDE.
I don't know what happened, but since I installed the latest update, if I setup a new silverstripe project as a php project, I get full code completion for sapphire classes, as well as documented popups for any class I try to call, including details of expectable variables. Not to mention full class and function lists and loads of other stuff you would get in a high end IDE like Microsofts Visual Studio.
I am getting the same with both Aptana on my Mac and my Linux box. I have tried other software to develop for silverstripe, but I am really rocking Aptana! I would recommend it for anyone looking to develop Silverstripe.
I will just post the link, then stop pimping the thing, honest :). http://aptana.com/