There's a user review on the lower right hand side of the page. Daniel Fisher from Sweetwater gives a little demo of the unit, mostly effects.
The main difference between the Digitech Vocalist 3 and 4 is that the VL4 can do up to 4 parts and the VL3 can do up to 3 parts, whether doubling or harmonies. Of course the VL4 is more expensive.
The only thing I will say is that Digitech has been making vocal harmonizers for over 20 years, so its nothing new for them.
Both Roland and Boss have introduced Vocal Harmonizers in the last year. The more expensive Roland unit includes Choir Voices that can be played on a KB in addition to vocal harmonies that the other units do. I don't know how valuable that is too you. I'm a KB player and I have plenty of Choir Voices on my Keyboards. The Roland unit is $599, right up there with the Digitech Vocalist Live Pro, which I can tell you from my own experiences is one fantastic unit. The Roland is a desktop model and the Live Pro is a rack model. I've heard the Roland and it sounds fine, but for the price I don't think its in the same league as the Live Pro.
As a guitar player you may want a stomp box vocal harmonizer. Before you buy anything I would suggest that you consider one of the Digitech units. Lots of bang for the buck and features you can really use. Roland and Boss are fairly new to this category and I don't know if they are up to snuff yet.
TC Helicon also makes stomp box harmonizers, and a series of primarily single purpose units like a doubler and there's pitch correction only box. They also make all in one units which is a better investment. Pay a little more and get more features that you will grow into as you keep using the stomp box of your choice.
Personally, I'm not a big fan of pitch correction, it "can" sound kinda phony. I'm not singling out any particular mfg that might sound phony, but generally the only thing that is usable is a very small amount of pitch correction. It can't make a person that tends to sing flat sound like he is always on key. There is no substitute for singing on key.
Which unit is a better stomp box for a giutar player ? the TC gxt or the Digitech vl3
on the demos the TC seems to sound more real but I like the idea of choice of male or female voicing. But a must is the ease of use during live gig do both units recogonize the key you are playing in or do you have to set manually before performance?