It's a spectral filtering sampler basically. Spectral filtering as an effect has its own possibilities and apparently it sounds good in Iris and Iris can be played in real-time.
There's nothing exactly similar (like real-time) to Iris in the market, but there are numerous other software that do somewhat what Iris does: Metasynth, Audiosculpt, Spear, Alchemy. Technically these have a lot more features than Iris, but being real-time and being playable is unique to Iris.
From these Audiosculpt and especially Metasynth are the most extensive, but they aren't "playable". Spear is less extensive in features, but it's free. Alchemy has spectral filtering and good sampling features, but its spectral features are a lot clumsier, slow and rather difficult to utilize compared to Iris.