This is a fun little book, worth checking out for anyone interested in language and particularly conlanging. The fair way to judge a constructed language is by its own intentions - and toki pona is about minimalism. I think there's some reasonable quibbles about this, especially insofar as the book is pitched as a mental health thing (personally I find eating with a knife and fork less stressful than a Swiss army knife!) but jan Sonja has succeeded in making a functional language with a very restricted vocabulary (a mere 120 words) and that is very impressive. The grammar is pretty easy when you get used to it, and although I'm not sure I could get into the habit of thinking in toki pona, I did enjoy the effort required to think about things differently - we might actually be better off if we didn't have a word for 'car', but had to describe it by reference to other things, that's an interesting thought experiment.
At the same time, I felt there were a few other aims which were less openly acknowledged. The language draws heavily on tok pisin, a creole from Papua New Guinea, and jan Sonja is clearly interested in trying to create a non-Eurocentric language. It's not just the vocabulary and grammar which is simple, but the phonology - just 5 vowels and 9 consonants which are all common cross-linguistically. That's cool because a lot of auxlangs are very Eurocentric, but a part of me feels like a minimalist language should also require minimal effort to learn - I don't know if it needs to be universal in its ambition, and we could probably make do with merging the grammar with vocab tied more directly to each language family. Some of the etymologies are easy enough to spot, but a bunch aren't, and a bit of a bigger sound inventory might have helped make those 120 words sound a bit more distinctive. A particular bugbear for me was the frequency with which single-syllable words like 'la', 'pi', and above all 'li' also cropped up in other words. Okay, maybe there's nothing in toki pona's minimalist goals to say that 'soweli suli li moku e kili lili' (big animal eats a little vegetable) shouldn't be such a tongue-twister, but it also feels easily avoidable. Equally, although this would change the language considerably, I personally find the Esperanto suffixing a more effective kind of linguistic minimalism than toki pona.
That all said, it is a nice detour from other language learning and I'd still recommend it - not least because relatively few attempts at conlanging are so clearly motivated. Although I didn't enjoy all the stuff at back - I admire the intention behind the inclusion of a sign language, but I don't think it completely worked - I did appreciate the insight into the broader philosophy behind the work that the example texts provided.