I've gone through several recently... and have an Eventide for sale in the classifieds! With that in mind, here's my opinion...
Other than tone, the biggest thing is, do you need midi capabilities or not? If you don't need midi and you're just using it for one key (like Em for example) - the old Boss harmonist HR-2 is your best bet. It tracks better than ANYTHING else on the market. Why? because it has a input that is used for determining the pitch. Harmonizers sound best in the loop, but then the harmonizer is trying to figure out the pitch of a distorted note. I used my tuner out on a wireless to feed the Boss pitch input, but had the harmonizer in the loop. Worked great. however, I needed midi because my bands play several different songs in different keys that need harmonies.
If you need midi (basically the ability to store different keys in the pedal) - it depends on what you really want. I am using a TC Nova system, works great for my purposes. However, the Eventide SOUNDS better. The TC is a bit easier to set up and for the few times I use harmonies, I'd rather have the extra $ than the Eventide.
Plus the Eventide does have some cool stuff like it can do arpeggios, synthy sounding stuff, FOUR intelligent harmonies plus your root note, all the cool detune/pitch shift stuff from the 80s that Van Halen and Vai and others used...
Here's a clip of me playing a Vai tune with the Eventide, one guitar. It was pretty fun.
https://soundcloud.com/okstrat/fumbelina-2-27
