There's an interesting post comparing the benefits of an agile process over a long drawn out one. (Disclaimer: the post is by the head of the Eclipse foundation, so there's clearly the potential for bias; although I don't believe it's overly biased myself.)
The net observation is that whilst Eclipse continues to bring out new milestones (Eclipse 3.2 M1 is already available, only just after shipping 3.1) that other tools are lagging behind, in this case, Visual Studio. In particular, the two communities tend to have completely opposing views; Eclipse users love their IDE, whereas Visual Studio users merely put up with theirs. There's a difference between use-it-because-you-have-to instead of use-it-because-you-love-it; specifically, the latter group are way more likely to contribute back towards the Eclipse community.
In any case, this is more an advantage for agile development processes per se, rather than the Eclipse IDE, but if you're looking to build a large project to go out to thousands of users, you might like to peruse the Eclipse Way presentation at the 2005 EclipseCon.