Convolver is great. I've simply been dropping in percussion samples (hats, snares, cymbals, etc), and using the impulse generator as a sound source. This can bring some life to the samaples, in that you can adjust the spectral content of the 'sample' by filtering or adding noise to the impulse, before it hits the convolver. Makes cymbals shine and sparkle more realistically than just dropping the same sample into Flexi.