Danny Danzi
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Some free ones are ok, but your best bet is to pay for a library. I like Pristine Space as my impulse loader or the Waves IR's.
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-Danny
This is the key with convo processors, the real differences are in the Impulses you use, if you have crap impulses the results will be rubbish, and it's worth spending the money on a decent library if you can.
The Numerical Sound guy has some excellent stuff he did for VSL he also worked on the Alicia Keys Kontakt library. Not cheap, over-priced perhaps but with libraries you'll get what you pay for.
Most of the newer convolution reverbs have turned into hybrids these days I guess because convolution only deals with the time constant aspects of the tail which gives that sense of lacking by itself and I'm not convinced by one particular verb doing what I want which is why I keep the convolution processing aspect simple (and free) and setup the chain to do the filtering and modulation myself.
If I had the money though I'd probably just plump for the Waves IR-1 for the convolution stuff and be done with it.
Oh, and there's some good material here if you want some impulse stuff to mess with.
http://www.openairlib.net/ The St Magaret's church set particularly is fantastically done with lots of mic positions, they are mostly B-Format files so if you are clever when converting to stereo you can have your impulses facing whatever way you like from whatever position you want in the room.
Here, they used these impulses to create a hybrid 3 dimensional auralization of the space computed along with the visual animation. Check the video tab.
http://www.openairlib.net/auralizationdb/content/st-margarets-church-national-centre-early-music