This was the first modular soft synth I ever bought, and honestly I still think it is the best. I recently bought Reaktor, which is indeed very nice and strongly supported, but SynC Modular's ability to do DSP primitives all the way down the simple math - and do so with surprisingly good performance - is unmatched.
Quite literally, you can do almost anything in SynC Modular.
Programming it is relatively complex, not even counting the fact that DSP programming itself is a black art. I have no way to judge how it compares to Reaktor in this sense, but my opinion is that SynC could have benefited from some UI improvements.
Alas, SynC is no longer supported. NI, in a brilliant move, hired Dr. Sync to work on Reaktor, thus killing the one product that really could have made a dent in Reaktor's market.
There are not as many publicly available ensembles for SynC Modular, which is a shame but is understandable given there will be no further updates to the program. So, if you want out-of-box experience, you probably will be happier with Reaktor. If you want to be able to prototype literally any DSP algorithm in a visual language, SynC is the tool for you. It's for the hardcore DSP/synthesist enthusiast, not really the general artist-public.
SynC is still my favorite soft synth, bar none. Reaktor 4 made a dent in SynC's position, but SynC still outshines it in my view.
Discontinued. Remains downloadable here (beware, the first link of the second column is wrong, use the link "SynC Modular Page is DOWN again." located at the bottom of the first column) and here but can't be used for other than personal experiments.
9. REDISTRIBUTION. You may further distribute the evaluation version of the software, provided you distribute it in the original unmodified form. You may take money for the distribution of the evaluation version to cover the media costs and other expenses arising in the process of the distribution.