Phaser is to the 70's what Chorus is to the 80's, such a great movement within your tone with the additional benefit of adding groove to the overall song. Back in the 70's the phaser to rule them all was the MXR Phase 90 due to use by EVH, however the popularity of that pedal encouraged companies such as BOSS & Electro-Harmonix to create their own versions.
Using the single knob model the Phase 90 & 45 laid the ground for, EHX added an interesting "color" switch allowing players to switch between a much more subtle and a more obvious tonal change effectively giving the equivalent of two pedals in one.
This is the V1 model, it encroaches more upon the clean tone than later models but has a more usable "color" mode. The speed controls spread ramps up somewhat slower giving greater control over lower speeds which is arguably where Phaser tones sound best.