I bought this after having seen a video of someone using it live (and then reading up on it). My thoughts were that, if it was half as good as people were saying, then it was worth trying... I've not been disappointed!
I play as part of an acoustic duo or solo, covering a few classics but mostly playing originals, and nearly every song has a vocal harmony in there somewhere.
The Digitech VL4 really makes it easy to perform solo and add the extra dimension of vocal harmony - and it's great for giving that BIG vocal sound when there are just two of us on stage too.
It can be used out of the box, with a pretty good selection of usable patches - of the 50 sample patches, I've found 10 or so that can be used live with no changes. But it comes into its own when you create your own sound patches - I took a few days to really work on these (I'll probably add a few) but I've settled about 15 harmony variations that I've created, with one, two, three, and four harmonies, which I can use for the range of songs I play.
Connection is simple too - connect the mic with a standard XLR, plug in the guitar with a standard instrument cable, route it to your pa via 1 or 2 XLRs or 1/4" cables (depending if you want stereo or mono) and off you go. You can bring the guitar into the mix so the stereo fx can be heard, or route the unchanged signal to a guitar amp/external fx pedal, etc.
For practising, I've used the unit as a guitar/vocal mixer, sending a mono signal (with one XLR lead out) to a powered monitor speaker. I've been pleased with the results. But it does sound best when used in stereo and connected direct to a PA.
PROs: Simple connections; useable out of the box; clear and intelligent harmonies; ability to set-up your own harmony patches.
CONs: Lack of on/off switch; no battery back-up; no pc connectivity for easy patch creation.
And finally... even though I've only had the VL4 a couple of weeks, everytime I've used it live (four times to date) people have come up to me an commented on how good it sounds, and how they want one!