I bought this for cheap (not that cheap really, it's US$18) at the supermarket just last week while it's selling at the same area at the Japanese restaurant I frequent for double the price! I've never tried it but I like kokuto (black sugar) and upon checking with the staff I know it'll be something like rum. In fact, it is made with rum.
The Japanese loves their Umeshu and it's been said to be made from steeping the unripened ume fruits in shochu (another type of Japanese liqueur) and sugar. Frankly, I don't remember liking Umeshu but I certainly love this potent drink! Yes, it's potent although it didn't feel like it at first.
It's best on the rock, with coffee or ice cream if you wish to be more adventurous!