I have to say that I found De Mundo incredibly interesting. It is clearly not a work of Aristotle, but it does utilize quite of bit of Aristotle's thought. De Mundo really seems to be almost a doxographical work. It reiterates portions of Aristotle's Meteorology, but also seems to contain extracts from Stoic and Platonic works. As it stands, the treatise appears to be the work of some Middle Platonist. It expounds a cosmology that really is more Platonic than anything else, but also seemingly influenced by various Stoics. Apparently, some have suggested that De Mundo contains portions of the writings of the Stoic Posidonius. That is an intriguing possibility. I looked through my Stoic reader and it seems to not just be dependent on Posidonius, but on a couple of Stoics. This edition also contains a wind diagram that was missing in Aristotle's Meteorology
De Spiritu was far less interesting. It does read like Aristotle and uses some arguments from the Parva Naturalia. It's discussion seems aimless and meandering and doesn't focus on respiration exclusively. It does reiterate Aristotle's belief in the primacy of temperature for life. The author seems to see some nutritive aspect of respiration, but he doesn't say a lot that is particularly interesting on the subject.
I give De Mundo around 4 stars and De Spiritu about 2-and-a-half. Overall rating 3 stars. I definitely recommend reading De Mundo ( )