First impression of Cthulhu Mittelalter is: "impressive." After quickly scanning the sourcebook (that's what it is), the second impression is: CMA is a lot of Middle-Ages and less Cthulhu. Let's elaborate on that...
The first 200 pages consist almost entirely background historical material, which spans 1000 years (from 600 to 1600 AD). Of that, the first 75 pages or so describe the world and the era. The remainder is divided in 3 chapters more focused on the life in the MA: worldy, spiritual, and occult. Mostly background material, sprinkled here and there with "Play Tips", known personae with CoC stats, and in the 3 last chapters of that part: descriptions of Koln and Hamburg, justicial systems, knighthood, Jews and Muslims, a monastery, inquisition, alchemy and famous occultists, a cult, and two castles descriptions...
Compared to the 20 pages of background info in Cthulhu 1000 AD aka Cthulhu Dark Ages (CDA), CM gives you 10 times more, which makes sense considering that CM spans 1000 years of history, while CDA does 100 (950-1050 AD). What is good though in CM and missing in CDA, are e.g. the city descriptions, the 2 castles descriptions (with maps), and the map of the Cluny monastery.
However, there is a missed opportunity here to not have deepened the connection between the background info and gameplay - room enough for that. To just to name one example, the descriptions of famous occultists (Alhazred, Flamel, Dee, Nostradamus, etc.) come without CoC stats, which would have been nice.
The final section of the book, i.e. the last 100 pages, makes up the gameplay oriented part of CM. It consists of 3 chapters: "characters in the Middle-Ages" (40 pages) and two original scenarios, "Servants of the Serpent" set in Viking Iceland and "Children of Darkness" set in a castle in Alsace, Holy Roman Empire.
The characters chapter largely borrows from Cthulhu 1000 AD. The real meat of the chapter is the expanded occupations section, which counts 40 occupations, doubling the CDA count. This is the best part in my opinion, with e.g. special attention given to gender (Nobelwoman, Priest's concubine, Wise woman aka witch), and Late-Middle-Ages gems (Alchemist, Inquisitor), three sort of knights, etc. A nice addition to top this off are 8 sample NPCs drawn from core occupations (pp. 228-230).
There are some more sample rules (movement, exposure, illness, poison...) copied over from CDA, and an expanded section on combat and weapons, incl. early fire weapons (pp. 234-243). A very nice addition is a special section on Insanity (pp. 243-246), which tackles the dilemma of how the Cthulhu-Mythos is to be perceived through the Middel-Ages lens. There is a short SAN-recovery section here from the CDA rulebook, which, with the Insanity pages of the background info (pp. 113-114), forms a kind of insanity-in-the-Middle-Ages primer.
Oddly, the Magic section is only two pages! It completely sidesteps the idea of a special grimoire adapted to the era (referring instead to the Mythos spells of the CoC core rules). There is a very cursory Alchemy paragraph, which, given the setting and the teasers in the historical section and in the occupations list, is a disappointment. It would have deserved proper treatment, at least a few pages of possibilities, rules, artefacts, etc.
Last but not least, there are the two scenarios. Proper review of those would require in-depth reading and playtesting so I'll refrain from that. Suffice it to say that it is good to have two solid scenarios. Cthulhu 1000 AD (and CDA) fell short by only offering one complex scenario. To this day, we still miss in CDA what The Haunting did to CoC.
So, my first impression. I still stick with "impressive" for the wealth of background information, the research, etc. It could have been even better if that part also included a description of a village, and more buildings (Palace, Church/Cathedral, etc.) to help the Keeper new to the era.
Having said that, I do find the ratio historical resource on one hand, and gameplay stuff on the other, too light on the game stuff (too little Cthulhu, too much Mittelalter?). Especially considering that half of the gameplay stuff is gleaned from CDA.
Missed opportunities? Plenty, for instance: more in-game hooks and rules spread through the background part to make it more game-centric and fire the Keeper's imagination. Alchemy should have deserved a place of honour. An expanded (Mythos) Bestiarium would also have been great. Moreover, skipping out on (Mythos) Magic is either a hard choice or a cheap omission. Basically I'm missing a solid Mythos-in-the-Middle-Ages chapter!
What I liked best: the wealth of information on the Middle-Ages (with maps and description of cities and castles), the occupations (incl. sample NPCs), the expanded combat/weapons rules, and two original scenarios.
If I had to redo it? I would merge Cthulhu 1000 AD and Cthulhu Mittelalter, take on the missed opportunities, and thus make the ultimate "Cthulhu by Torchlight" supplement + sourcebook.
To conclude, here is an overview of contents expressed in %:
65% pure historical material, some of which can be more or less directly ported to gameplay. About 5% of that are play tips (typically: a reference to a published scenario) and film tips (e.g. Go see the 13th warrior).
15+% rule material, mostly centered on PCs and weaponry. Of that, about half is copy pasted from CDA or my patch, and the other half is genuine new material (largely new occupations and weapons).
15+% the 2 scenarios.