I agree. Pure fruit juice, like the grape juice you would get at a winery, is probably in need of some tweaking (above all the sugar/SG).
However, it is posible.
In stating that, I would probably do a 5 gallon batch this way.
4 gallons 100% fruit juice
2-3 cans of grape concentrate (like Welch's - for body - fruits tend to be a little flabby, depending ont he fruit you choose, though)
Sugar to bring the SG up to at least 1.075.... That's depending on the sugar content of the fruit juice.
campden tablets (5) or k-meta. Pectic enzyme - throw these in there and wait 24 hours.
energizer 1.5 tsp, nutrient 1.5 tsp, possibly tannin 1/4 - 1/2 tsp.
I think I remembered everything...
It really depends on what you make. Strawberry juice would be a bummer, I think. Rhubarb would be bad too. Apple? Definitely you'll need the white concentrate (Welch's) for that. Pear? Ditto. I am not too sure about blue/blackberry. I made some that was really hot, and I thought it could be used in the next NASA rocket-launch.
Hope this helps.
Martina