Concise rules, yet contain great design notes about why certain choices were made, and variants for you to consider. Great tables that I will likely find myself using in other games as well. Knave is definitely on the top of my pile for go-to systems.

The good:

• It's big. Lots of items. Even though each item sprawls across multiple (2 in the majority of cases) pages, the page-count means there are still a lot of items.
• If you like background and flavour for the items, this has tons.

The bad:

• Needs another editing pass. Typos ("it's" instead of "its" in many places), the core terms are not strictly used (e.g "Dexterity Save" instead of Dexterity saving throw", or "Athletics check" instead of Strength (Athletics) check" as is the core convention), and the style varies from the core books (e.g. the capitilization core terms, italicising of spell names, etc.). This may be a minor point for some folks, but I'm disappointed when products (expensive ones, especially) lack this attention to detail. If those details don't bug you, add another two stars, but for me it's the difference between an okay product and a great product.
• I still need to go through all the items, but I've already seen some with problematic mechanical interactions that I could see needing DM rulings to resolve at the table.

The book does contain a lot of interesting mechanics. This is both good and bad. Some look quite fun. Others might be difficult for some players to understand and/or remember, so that might be consideration for some tables.

If you imagine a triangle with D&D, Dungeon World, and OSR (Black Hack, etc.) at the three corners, ICRPG will be smack in the middle. It runs simple and fast like DW and OSRs, but has just enough turn structure to be close to D&D without getting bogged down in feats, and spells list.

The GM advice and ideas on encounter building are applicable to nearly any RPG, and are a treasure-trove of ideas for any GM.