I tried BitSpirit a long time ago. I can't say there was anything particularly wrong with it, but it didn't convert me from Shadow at the time. Only Azureus managed to make a convert out of me.
But have a look at
XBT while you are at it. It's fairly bare-bones at the moment but it looks very promising. It's written in C++ and uses it's own engine, i.e. it's not based on BitTornado.
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