This pedal is a pantheon in rock music. Chances are your favorite rock music is laden with phase pedal effects. For example, Led Zeppelin's "Ten Years Gone" -- that first hard rock lick in the beginning, there's some serious phase. Phase is excellent for most instruments, especially bass guitar, electric guitar, keyboards, even voice (probably from a rack module, though, unless your singer wants it live). As you can tell, phase is used when you want to add a 'washy' or 'warbly' tone. The box grabs the incoming tone and 'phases' the wave at a different rate and tone then feeds it out. The speed can make it seem very gradually whooshing or high speed for a sound similar to when you hum and tap your adam's apple. And for good entry level and pro type phase, in my opinion, this is an absolute must. These are the pedals Eddie Van Halen somehow turned into an incredible tonal fury. For funk, reggae, R&B, uptempo grooving and rhythm, this pedal is simply unbeatable. Try a Les Paul with the tone high on the neck pickup, phase it out at about half-way on the speed dial, and try a couple of 'bob marley' rhythm chops...home free, mon!
There are many companies manufacturing phase; it's a common effect. Even MXR tried to outdo themselves with a Super 90 which was a wider orange box, adding some different wave settings. Why doesn't it sound as rich and full as the simple 90? Who knows. There's no overstating the versatility of this pedal, especially when handling any kind of rhythm work on your instrument. Soloing with the pedal makes the notes sound too washy for my taste and I have a tendency to use it primarily in rhythm. And it doesn't mix well with a wah-wah (too much loss of control in cancellation). But for the $100 to $120 you'll pay for this pedal (maybe even cheaper), you'll be adding a very rich tonal variation to break up the thin tone of no effects.
This is one of my favorite pedals for any rhythm work in funk, rock, r&b, reggae, proggy/psychedelic. One of the great grandaddies of killer tone and reliability.