I only have experience with Samplitude's IR reverb called 'room simulator'. It is the first software reverb I've even considered close to hardware, sounds very good considering. But, this comes at a steep performance price.

IMHO, IRs are WAY better than software reverbs... the only things I've noticed are a) to my knowledge, you have to match bits and freqs (you can't use a 44K/24bit impulse on a 48k/16bit recording)... and b) in the PC world there aren't many reliable ways to use them... yet.

Also of note is how they work... software reverbs create the sound through calculations... hardware units do basically the same thing, just a lot better... but with IR's you more or less record the sound of the reverb, just like you would a guitar or anything else... actually, to be more specific and accurate, you obtain the sonic imprint of the unit or room.. which means it should sound *very* close to the original room/unit...

but, it also means you can't tweak the sound as easily... in a room you can move mics around, or with a unit you can adjust room size / density / dampening / etc. ...with an IR you can't do all of this, though you can usually tweak some things (i.e. wet/dry, pre-delay)...

so... the Lexicon 960 IR could be very close to the same quality as the hardware unit depending on how well the image was captured... but with less functionality it may be harder to dial in some sounds that you may want... (as always, others please feel free to correct me if any of this is wrong)...