Among the best speakers ever, in my opinion. They used a crossover design patented by Richard Mondafieri of Mac tuner fame that created extraordinarily steep crossover slopes (hence infinite slope) without the ripple and phase side effects normal with steeper slope that allowed each excellent driver to function only over its optimal frequency range without interfering with the output from other drivers. The sloped front time aligns the drivers. Its surprising what an effect this combination had -- very good imaging and an almost total absence of boxiness for some reason.
John Sollecito (hence JSE) who ran Infinite Slope used first rate drivers and the enclosures were very well built, though the veneer on mine was oak and was rather fragile at corners. There were some variations in the speakers as drivers became available or not, and the versions with the Dynaudio/Vifa/Focal drivers seemed a bit better than the ones with SEAS drivers, but that's an impression based on widely differing systems and over a long time difference.
Generally speaking, a somewhat warm, very detailed speaker with excellent dynamics and imaging.
Don't judge them using mediocre amplification -- revealing speakers tend to sound just as bad as the equipment they are used with.
Later addition: After John Sollecito folded up JSE/Infinite Slope, he founded Source Technology, which made well regarded speaker with conventional crossovers, and the infinite slope patent was licensed to Joseph, which made very highly reviewed speakers also.