Its pretty much equal to RO water. I've read a couple people claim that the copper lining can leach giving your plants copper overdose if not alternated with RO water but not too sure on that.
Even though the PH is seems low, it has very little buffer so mixing it with any amount of tap water or pouring it through your soil will raise the PH easily. It is only a bit of carbonic acid causing the ~5.5 ph reading, same exact situation with R/O and distilled water.
Recently I have been checking to see what happens when I put pulverized dolomite lime in my R/O jug. I am hoping to get some calmag from it. It raises the PH from ~5.5 to ~7.1 but I don't have an EC meter. It might be possible to get even more calmag dissolved by adding some plain peat and letting it stew. Verdant suggested adding citric acid and letting the lime neutralize it.