I'm more interested, actually, in finding out the value (to Gilbert) of this spiritual journey than finding out whether the book is well-written or too boring or what. 

I haven't read this book, so I don't know whether the following applies. But I can think of one circumstance when I would feel justified calling into question another person's account of their spiritual process, and that's if by their own account they find truth in traveling to exotic lands and absorbing ideas and experiences emblematic to them of foreign mysteries, but don't actually do any self-reflection or integrate those experiences into everyday life in a meaningful way. That's not personal growth or a relationship with the Divine, that's Orientalism--fetishizing and commodifying the foreign. It leaves you right back where you started, only in a sari. That's exactly what the NYP is saying in the quoted passage, and the criticism might not be without merit.

On the other hand, just because someone traveled a lot while sorting out their cosmic questions doesn't mean the answers they found were shallow. It might have been a quest for exciting "peak" experiences, but peak experiences can still lead to growth. It's all in whether you actually do the work or not. And since I haven't read it (and am not planning to) I don't know which, if either, of these situations apply.

I've been thinking lately about those of us (I include myself) who are tempted to make our religion a religion of peak experiences. The peak experiences are so great, so bright, so captivating, that everything else seems like a distraction. But all that "everything else" is the field where the real work is done, and where the spiritual insights we find in prayer or on a mountain in Nepal actually have a chance to turn into something of real value. It's easy to forget about doing the work to spend one's time hunting down the next peak experience instead.