While Quartz Composer has come a far way, insofar as actually being able to do generative imagery, I think it's had a bit of a slow evolution in that regard. Kineme is actually the standout company in providing tools to actually generate substantial imagery within QC... perhaps the only company? (Things like image filters, importers, and other things are extremely important, and my comment isn't to deride those tools at all, but to point out that tools that generate quality imagery are a bit lacking in Leopard. To be fair, I don't believe that generative art was any kind of thrust of QC at its inception).
Though I do use many generative techniques in QC, I often like to take the approach of feeding it outside resources: video, music, still images, 3d models, web feeds, and so on. That being the case, I have a ton of other "things"... hardware, software, to create the sources that I manipulate with QC.
Gimp is a go-to tool for me when I work with QC. If for nothing else, the mask color to alpha function is something I use constantly for preparing art for Quartz Composer. Sorry, but something about "Gimp" always brings Pulp Fiction to mind. Youch.
I like to check out different OS's and MathMap was something I was familiar with when using Gimp with Ubuntu Linux. So I recently went to download a plugin that would work on the Mac OS X with Gimp, figuring there would naturally be a similar plugin for the X11 environment. To my surprise, MathMap for Mac is a standalone Cocoa app!
I find some of the filters in there to be really interesting, and not quite available with other systems. Droste effect is a killer one, and is actually the main reason I ever tend to break out MathMap. I believe that it's also available as a Photoshop/Adobe plugin, but don't quote me on that. I seem to remember seeing it in a pulldown menu while working in Adobe CSwhateveritscalled.
I also noticed two things that had eluded me before on the project page (http://www.complang.tuwien.ac.at/schani//mathmap/) - that the XCode project for the MathMap app is available, and a film clip that demonstrates something called MathMap composer... which looks as if it is part of the "unstable release", of which there is no OS X variant of yet. It looks a great deal like Max/Jitter style nodal box stuff to me, moreso than QC... but similar to both.
Some of the filters are nice for getting some interesting pre-prepared art going to use in the Quartz Composer environment. I might mock something up with Gimp... maybe total Gimp line drawing, or using Gimp to treat a scanned in image, then put it through MathMap. Would I rather have the filters available in QC runtime, to be able to manipulate on the fly? Of course! In fact, some of them can be implemented in QC, and it's just not obvious. However, things like this make me realize that sometimes it's the interface and controls that are available that lead to "cool stuff". Some of this stuff is totally do-able in Quartz Composer, it's just that it's not implemented in Quartz Composer in an obvious way, so people don't do it. Ultimately, MathMap doesn't get a big win from me, because I can't use it for video, but it's still cool for stirring around brain cells.