As Ventura noted, there is some redundancy when you own all of the eventide 'factor' pedals, but the ModFactor alone can do so much. You can program it to be (or emulate classic versions of

various choruses
various phasers
various flangers
various tremolo or leslie effects
a chopper (and isn't stutter editing all the rage these days? ugh...)
various wah emulations (add expression pedal)
an auto wah
Aside from the obvious use of the expression pedal to control the wah filter, that pedal can be mapped to any function on other patches - such as mix level to bleed in the wet/dry or sweep depth for choruses, chopper rate (1/4 notes, 1/8 notes, 1/5 Tripplets etc.) and on an on. Also the unit has tap tempo or you can take a clock from your keyboard player and lock in your mod rates with the rest of the band.
I'd also highly recommend buying or building an extension switch (3 extra switches) as it accesses more functions and makes the bank up/down easier to access.
Of course, it only does one of the above effects at a time, but does it in stereo, and can run at instrument level or line level - so in front of your amp or in the loop. So much bang for the buck in the Eventide stomps, but RTFM or else you'll miss out on its best features.