As usual in a collection of short stories, you will find that you like some better the others and there even be a few which you simply did not like at all. It is largely a matter of personal taste. As far as I am concerned, I found that 7-8 were superb, another dozen were good to very good, and there were about 3 that I did not like. I will refrain from telling you which ones and why, partly because other reviewers have already mentioned their own preferences and partly because I do not trust myself to avoid spoilers.
My three favourite stories were Power Armour: A love story, where a man from the future has to remain permanently in his power armour to prevent him from being murdered, Don Quixote, which takes place towards the end of the Spanish Civil War, as the Republicans are about to lose, and the rather horrific and nightmarish Helmet where human beings are imprisoned into powered armour and commit atrocities. However, this does not mean in any away that the 20 other stories were not good. It only means that my personal and very subjective preferences went to these three.
As hinted to above, I very much liked this collection and most of its stories. However, the higher the number of stories included, and the higher the chances that there will be at least one of them that you may not like.
Accordingly, since it is unlikely that someone would like ALL of the stories and think that they are all wonderful, it is also somewhat unlikely that the book deserves five stars. For me, there were three weaker stories. This is less than 15% of the total number, implying a four star rating...