sure. I got 100% charge a couple of times (kept retesting), originally from about 80% on the iphone, I think. that took the Solio down to ~60% charge. when I first tried it -- and it didnt work -- my iphone battery was flat (would only come on to say 'plug me in'), so will be interested to see what happens now. (will post back.)
btw, I may or may not have had entirely factory firmware the first time round.
oh and if you do a backup first (which iTunes seems to do automatically whenever you sync anyway; but right click and hit backup from the device listing to do one explicitly) a firmware restore is actually quite painless and only takes maybe 20mins or so. I've done a few now.