I purchased this a year ago as a graduation gift to myself, and this book is literally worth its weight in gold (I mean it. I purchased the hard copy). I originally found out about this system when I was watching Professor Dungeon Master on his Youtube channel Dungeon Craft and he mentioned something out of this book, like the target numbers or other (eventually the Professor started to have multiple videos where ICRPG was referenced). The more I found out about this system and the author's philosophies on RPG's the more I gravitated towards Runehammer's systems. I, like many others, got an actual introduction to D&D through 5e (prior I was a plebe who grew up thinking D&D was a social suicide device; what can I say it was the 90's and 2000's), and quickly landed the role of DM amongst my friends. I found that too much time was being wasted in combat that was bloated and that could slog on for hours, rulebook lawyering, and trying to create challenging scenarios for suped-up PC's under vanilla 5e. I just felt like there was something out there that could bring the joy of playing and make life easier for the DM (which helps keep the game going), while still giving the players that challenge and great excitement of playing (which is also necessary to keep the game going). I found that missing something in Runehammer's Index Card RPG.
Essentially this book, whether you use its systems or not, breaks the shackles of what is possible when DM'ing. The GM section alone is worth the price of admission, yet it comes back at you with an overwhelming wealth of content, especially with the added digital art work of the index cards themselves that you can print (which his method of using those cards to create stories is ingenious and works very well win my games). The lighthearted tone and pure love for the hobby felt in the writing makes this an excellent read that is full of vibrance.
Do yourself a favor and purchase this book. Even if you can't ditch your 5e or Pathfinder game (by your choice or your players), this book will make your life easier and fill you with vigor to go out there and "be a beacon of camaraderie" and "a terror to behold" as your players laugh and quiver with the experiences this book will help you create, as it sharpens and hones your skills as a game master.
P.S.- Runehammer's online community is great for generating/borrowing ideas from like-minded DM's and players. The one negative I need to address in this book, is the fact that the hardcover for Core 2e does not come in white and red like Core 1e. Obviously personal preference with how the book looks on the shelf. I just think it stands out more and looks cooler than the black. Yeah, that is my only problem with this book. I hope this helps convince at least one person to make a great decision and throw your wallet at this guy.
P.P.S- Why did it take me over a year to think of coming on here and writing this? So ungrateful.