I'm giving this a 3 star rating for now and while it may seem harsh, here are my reasonings: 
 * Osuma uses humor rather childishly in the book, which more often than not, is corny and completely misses the point, such as references to New York when likening a city in the story. I don't see the point of such comedy or odd analogies, and how it contributes to the overall style and story (if it does at all). While unique, I personally find that it detracts from the central message the joke was trying to convey in the first place 
 * I find his drawing rather jarring for more reasons than one. It takes some getting used to but all characters that appear in the book tend to be topless and some of the males - completely naked. It is a style of drawing and expression, and it is certainly worth exploring as to why he chose to do it that way but until I find that answer - it will perhaps remain a rather bothering conundrum. 
 * About 94% of the content within the 1st book has got very little to do with the birth of Siddhartha. It almost makes me wonder if Osamu was employing the usual anime and manga tactics of "filler" episodes/volumes to stretch the story. The only way to know if any of this is interconnected to the actual story of the Buddha remains to be seen - after which I'll be glad to come back and amend this rating to reflect my view more accurately.