Well, I've stated before that I tend to use the harmonies more as a "feature" of the group sound than a backup. I am going for that "Four Freshman" / "Manhattan Transfer" type of sound. For that reason, and coupled with the fact that I still play a lot of left hand manual bass parts - I NEED to trigger the harmonies from my right hand. ALWAYS. It's simply a must have, and NOT an option.

The Ketron is the closest to the Digitech system in this regard because it can recognize chord structures from ANY area of the keyboard. The trouble with Solton is that the harmony notes will shut off as polyphony is exceeded too. I want a simple chord signal to be sent out to trigger the harmonies. I don't want the actual notes played to do the work.The Yamaha system can follow either side of the keyboard split, but ONLY in vocoder mode in the right hand. That is too limiting because I can't always finger the exact voicing I want the voices to play without affecting the kb sound. Korg will only follow the arranger side of the board - no matter what. (yuk)
A sequence user can pre-record the needed harmonies into a track (like the Tune1000 products), but that has always been an option, and is not hard to do.

S0 - what does all this really mean to a singer?
For starters - it limits how the chords need to be played on the kb.
Next, it makes it impossible for a "harmony solo" over drum accmp. only because the arranger notes need to trigger the harmonies. Think of Doby Gray's
"Drift away" - I like to open up the chorus section as an accepella feature, and this can't be done on the 9k without holding the chords down.
The way the Digitech works - it simply receives a chord on a specified midi ch, and holds that chord till a new one is sent. The on/off sw brings in the harmony voices, and all's right with the world. Very smooth, and the voices are always ready when needed. On The Yamaha, sometimes they need a split second to recognise the chord after the on/off sw is pressed - very noticable - can't deal with lags in performance.
In the Yamaha line, the vocoder method is too restricting to my chord voicings, and the vocal notes drop off if the sustain pedal is not held down forever. That makes for a very MUDDY kb sound.

If you're vocal needs are simple, and you are not a fussy singer, then the on-board units can help by thickening up a lead. If you are a serious singer, who wants to really "showcase" the harmony arrangements - the kb versions are still a step behind the leaders. Digitech is a great choice for value, and I've read that the Helicon unit is also great, but expensive. (and it's a two space rack unit)

I've been using the harmonizers since they first came out, and I have been very happy with the way they worked. This new add on unit is a small convienience, but at a serious cost. I can't reccomend the "built-in" over the "stand-alone" unless you allow for loss of quality. I'm pretty fussy about the vocals, and I don't like to cut corners where my voice is concerned. It's the priority of ALL my shows, and that's why I am so fussy about it. If your vocal styling needs are not as important to you - the built in units are an OK tool. Far better than nothing at all.