There already a lot of good points regarding the Scarred Lands in this thread, but I would like to add a thing or two nonetheless.
I had been DMing a campaign in the scarred lands since last spring, and I find it very enjoyable.
One of the things I like best is that the whole campaign is united by a common theme. While a lot of other campaigns feel like a patchwork, you always get the feeling that every place within the scarred lands does has it's place and it's connection to the rest of the world. So, even though there is a great diversity within the world, with places as different from each other as the necromancer city Hollowfaust, the empire of Calastia or the Blood Sea, which is tainted by the blood of the Titan Khardum, it all fits together. This diversity that is nonetheless connected by a common feeling makes the world very enjoyable, and helps building up suspension of disbelief.
Furthermore, the Scarred Lands have a very rich mythology. You get the feeling that the designers assigned a higher priority to mythology and legends than to numbers, statistics and cold hard facts. This feels exactly right for me. After all, I enjoy fantasy- and this fantasy comes to life if I can get a feel for all the fantastical elements. Details about economical background, demographics, etc., while interesting, come secong.
This is also one of the reasons why the Scarred Lands sourcebookds are such a fun read- reading them, I found many, many secrets, mysteries, adventure hooks, etc. Just reading the first gazetter for instance made me want to DM this campaign setting.
Speaking of secrets- have I mentioned that often you are left in the dark when it comes to some details of the world? You are, and that is a good thing. Reading the books, you get enough hints regarding what happened to the Slarecians (a mysterious species of the past), and what they were all about, but you never learn everything- leaving you enough space for your imagination to run wild.
Furthermore, there are a lot of elements about the scarred lands which are truly unique to it and which can be found in no other setting- the blood sea, the Slitheren (my new favorite Specie) and many other elements come to mind.
The sourcebooks are well written, and many of them offer enough material to run an entire campaign on. One example would be the Mithril sourcebook, which doesn't only give informations about the city itself, but also about the very complex surroundings, like the blood sea and the plains of Lede. Believe me, leafing through it, you will find tons of adventure hooks, ranging from city-intrigue, to social interaction with Orc, to downright dungeon-crawling.
Well, so much for now, I gotta go back to work. Incidentally, I would like to pimp my Scarred Lands story hour as well: You can read my personal campaign logs so far there- just click on my link.
Glad I could convert so well on such short notice. 
