I did use Swinsian for awhile. While, in broad terms, it satisfied a lot of my requirements in terms of pure functionality, I still found myself frustrated by certain parts of the interface. My criticisms of Swinsian would mostly boil down to small aspects of the design, usability and polish, which, taken collectively, made the experience of using it frustrating and subpar, at least for me. I wanted something that worked just as elegantly and looked just as nice as iTunes does (or used to).
The simple fact that Swinsian doesn't have an equalizer, for example, makes me really wonder whether the developer understands the needs and desires of the power users for whom Swinsian seems to be designed.
Also, an open-source media organizer run by a communicative/responsive developer is much better in the long term than a closed-source, totally proprietary application, in my view.
Postscript: If RAM and CPU use is of any concern, you might be pleased to find that jmc uses significantly less resources than iTunes, with generally superior performance. My personal library numbers around 30,000 tracks; memory use peaks around 200MB when scrolling through the entire library and generally hovers around 100MB.