It takes some time to get used to, but systems that support image-based development, in the tradition of the Smalltalks and some Lisps, can boost programming productivity a lot, by allowing very tight feedback loops. Combined with a good debugger it makes much easier to diagnose and recover from problems, and iterate over your design quickly. To experience the state of the art on this front, I recommend Pharo. There'…
Pharo is more of a research experiment than anything practical than can be used to engineer complex systems, I know I tried. It comes with many flaws, first and foremost being a lack of vision (often seen in academia) that significantly holds back its potential. When it comes to Smalltalk, I find Squeak to be a far more grounded and coherent experience. As far as state of the art in image-based development goes, that…
After spending a lot of time working in both systems, I unfortunately have to agree with this. However, the GToolkit folks (built atop Pharo) do have a pretty coherent vision and their systems shows it.
The biggest problem with Squeak, to my mind, is that not enough people are working on it. They are simply shorthanded. Even now there is a VM optimization ready to go that would improve performance 3x (SISTA), but it just needs a couple of months' work from what I understand. Vector graphics is another area where there could be worthwhile improvements. Juan Vuletich seems to be making the most headway there all on his lonesome, and while he's made a VM plugin, his work is really only for Cuis Smalltalk and not Squeak.
Still, I've tried to push more and more of my routine work into these environments because I want my computer to be more like them and less like a Unix-with-lipstick.