I have been practising on my 8-strings guitar made by Tihamer Romanek in the last three days. Mr. Romanek, he is a Hungarian harpsichord, guitar, lute and lute-harpsichord maker. It's much better to play scales, plucking or arpeggio exercises on this guitar than on a normal classical 6-string guitar. Practising on 8-strings is a serious physical work for the left hand since the neck is wider. Playing chords takes a long a time to learn on it but then it is so nice like on a piano! You 8 notes at the same time.
And it trains your technic much more. The instrument is more heavy than a normal 6-string classical guitar.
Doing lute, piano or cello transciptions is easier on 8-string classical guitar because you have more strings more range of sound. You can tune it on many ways. I tune it as normal guitar tuning (E-A-D-G-B-E) plus A (bass) underneath and another A at the top. Others normally tune it as a lute or a normal classical guitar plus two more bass strings.
This is not my 8-string guitar but I have exactly the same type - I got this picture from Romanek's website.