My review is in Scrye #104. Very, very positive. It's an amazing setting if you like classic campaign settings like Greyhawk, Mystara, Wilderlands, Aerth, or early Forgotten Realms. Of the campaign settings currently available, I put it on about equal footing with the Wilderlands (and note I say that when I will be publishing my own products for the Wilderlands)! It comes down to matters of DM style.
With Known Realms, you get the big picture on a vast canvas. In detail it is somewhere between the Greyhawk boxed set and the original Forgotten Realms boxed set. It has cultural differences along the lines of Mystara, but with a more realistic geography and a hint of Gygax's Aerth (the Known Realms is even named Aereth). As compared to the Wilderlands boxed set (same price), which gives you minute details about tons and tons and tons of locales and encounters.
Essentially, if you like having a campaign setting as a broad backdrop or if you like building your own elements within a pre-constructed design, Known Realms is for you. If you want something where you can pretty much just pick up and play, you would want Wilderlands. But then, with the Known Realms, you also have the Dungeon Crawl Classics, which tie in directly to the setting and give you plenty of pre-designed adventure goodness.
Bottom line is, IMO, Known Realms is the best classic broad campaign setting available on the market today. Wilderlands is the best setting for the minutiae. If you can get both boxed sets, you can use the cities, towns, villages, ruins, and encounters from the Wilderlands to flesh out Known Realms.
Oh, and if you have Ptolus, you could even work that in, as the lands to the far east (the Known Realms "Orient" is actually in the West). It would take some work to fit together the mythoi, but for cities, Ptolus can't be beat (and I say that when there are few bigger fans of the City State of the Invincible Overlord than I). Wilderlands + Known Realms + Ptolus = Super Mega Kamehameha Campaign!
