Things have been a little quiet on planet.sympy.org, but the less we talk, the more we work. :) With 468 svn commits in September and 89 in October (till October 5) , we are working hard. We made a new release yesterday, where we simplified the internals of SymPy a lot and I also fixed the limits finally.
What remains is to fix some problematic series expansion (that also makes some limits fail, but the limit algorithm itself seems to be bugfree so far) and then I'll concentrate on making SymPy more SAGE friendly. SymPy will remain as a small Python library, doing one job and doing it well. But when used inside SAGE, it will know how to convert expressions to and from SAGE, so that people can use SymPy from SAGE ideally the same way as they can now use Maxima.
I think there are already things that SymPy is better than Maxima in terms of pure functionality, like pattern matching, limits (after I'll fix the series code, hopefully in the next release). There are however things, where Maxima is better, it still has a lot more features (more integrals, more differential equations, tensors) and it's faster. Except for the speed, all the other things are really easy to implement in SymPy. The main advantage of SymPy over Maxima is that it is written in Python and it is really easy to implement new features in it.
A huge deficiency of Maxima is that it uses lisp and it's own language for doing the calculations. SAGE fixes this problem (but it doesn't support all features of Maxima yet, for example substitutions of subexpressions, that is needed in the Gruntz limit algorithm), so SAGE is in fact like a platform, a common ground for all these mathematical libraries, allowing them to be used using the same interface and thus one can compare the real features (and speed), instead of some possibly good libraries being disqualified just because they use an unfortunate language/interface (for whatever historical reasons), which is great and I want SymPy to compete in the same ground as well, because only this will tell, if it is good or not and which things to improve. Competition is very healthy, it's always good to have a choice.