The learning curve is quite short (and shorter still if you've used Eclipse or any other Eclipse-derived IDEs), and the code completion tools are efficient - and also work with any libraries you're using. I've been using it to do some AJAX and JSON development for an article I'm working on, and I have to say it's really rather decent. It's helped me track down some rather silly formatting errors I made while I was writing code to work with Flickr's JSON search API, saving me quite a bit of time. I also found the Outline view helpful as it let me quickly hop from function to function (and see what was calling what - and when).
A recommended tool.