Needed to add intelligent harmonies to my Kemper for live use - one of my bands plays some Boston and other tunes, and while our keyboard player is awesome, I'd rather have a harmony effect than a keyboard 'guitar' patch. So... here's what I've tried and what I've decided to use.
First was a boss HR2 Harmonist. Before you turn up your nose, thing works great, especially since it has a jack for pitch detection that you can hook up to a 'pre distortion' source. I ran the tuner out from my G50 wireless to it, and the tracking was PERFECT. However, no midi means that I was having to change keys and such manually, and on a dark stage? total pain in the ass. If you had really simple needs for a harmonizer though and didn't mind having to change settings manually, this is a great budget choice.
Also have a Boss PS5 super shifter. I prefer the harmonist above because of the tracking. The PS5 has some more goodies though like whammy mode, you can hook up a controller pedal, etc.
Have an Eventide Pitchfactor. Sounded great, damned thing bricked when I ran a Eventide provided firmware update following their every step. Going back to Eventide for a 'fix' and when I get it back will likely sell it. very deep box, sounded great I just became irritated with the update process and their tech support's way of dealing with a problem they caused. Box did a lot of stuff like synth tones and a arpeggiator along with the intelligent harmonies, could do super deep detunes, emulated some classic Eventide boxes, etc... a little pricey though.
What I am using now: TC Electronic Nova System. Tracks great, plenty of controls for each harmony (two total, but can pan each, add delay, etc) and since it's a multi effect, I also run a very short analog delay with it to fatten up the tone without having to 'use' my delay in the Kemper. can do whammy effects, the detune sounds awesome, Just a terrific box and even new they are IMHO pretty well priced for what you get - I saw some on ebay for just over $325 or so. It's a little big, so you'll have to find space for it or run cables from it to the Kemper, but so far two big thumbs up from me. Uses MIDI so you can change patches with your footcontroller seamlessly. I set up my boston tones and it's a winner.