5 out of 5 rating for Conan: Adventures in an Age Undreamed Of
I am a recent convert to 2d20, having played and GM'd it with Conan and GM'd a number of times for Star Trek Adventures. The iterations of the system between the two games are different, with the core mechanics and processes the same, but the level of detail (read: crunch) considerably greater in Conan than in the more streamlined STA.
Like most gamers of my age (late 40s) I am familiar with Conan as an idea and a pair of fair-to-poor Arnold movies. I started to read some of the stories only last year, as my interest in this game grew after playing the Free RPG Day scenario. Over the last 6+ months I've read a heap of Howard's Conan stories and have a much better feel for the genre and his world in particular. Thus, I approached the gamebook from the perspective of someone more specifically familiar with Conan canon, and as someone familiar with the system.
I think it's great. The book can be divided into two parts: everything related to the system as it creates structure for the game; and everything that presents the massive background to the world. I am impressed with both. The system, while certainly a crunchier version of 2d20 than found in Star Trek Adventures, is not complicated. Yes, there are many options available to players, especially when considering how to use Momentum, and the GM has to be creative and thoughtful about spending Doom. The core mechanic is simple and extremely flexible, determining success or failure, as well as degree of either, with the added bonus of making room for complications that can happen on otherwise successful rolls. This is very similar in function to FFG's Star Wars; however, I think it accomplishes these ends in a more simplified fashion, and the menus provided for Momentum and Doom usage make using those resources at the table an easier and faster affair than I found with FFG's system. I have described the system's learning curve as being not so much steeper than d20 (for sure), but rather longer. It's not inherenly complicated - there's just more to learn in order to become truly proficient as a player, so that you can play your character to the utmost potential within the story.
The other part of the book, all the world background, serves as a primer to those who are like I was a year ago: I'd heard of Conan but had never read the source material. The background on the places, peoples, and ideas of the world is deep and broad, punctuated throughout by great art and sidebars providing commentary on Howard's ideas and other issues related to conceptualizing the world with integrity.
The book is also a complete volume for gameplay, with full character creation rules, a bestiary, all that background information I just mentioned, a detailed GM section, downtime rules, and on and on. The single volume is all you need to run the game, in full.
If you like fantasy and are interested in a change from the norms of default DnD, and are also interested in a system that provides more options - albeit requiring a little more time to reach proficiency with it - you should give this a try. Modiphius sells the PDF directly from their site, and if you are only thinking of playing it you can get the Player's Guide, in PDF, from them for about $7. That book is just over 100 pages and is a page-for-page reproducation of the entire character generation rules plus the basic player-facing rules for tasks and combat. This makes for a really low barrier to entry for this system: a GM who has the main book, and players who each just get the PG PDF. Very clever production line, I think.
May 2019 Update: After running a number of trial sessions for my home group, we decided to pick up Conan as our regular game for a while, and are loving it. Yes, the system has more moving parts than 2d20 for Star Trek Adventures, and certainly more than for 5e. It is not, however, what I would call "crunchy" in that you have to keep grinding away at it to make it work...it's more...crispy on the outside and smooth within. Silly metaphors aside, the many moving parts to combat and magic are all coherent, and the core mechanic is clear throughout. As our group has become more proficient with the rules we've realized that we're adding more of the detail rules in each session - almost as if we started with only a basic version of the rules, got that down, and progressively enriched our play with more of the full system as time has passed.
Because of the many combat options, there is great opportunity for meta discussion at the table, making combat more like a co-op board game in some ways than a traditional rpg, in which many players check out when it's someone else's turn. Momentum spends make for a fluid economy of assistance and ideas around the table, which has enhanced in-character play. What it feels like is a group of adventurers who know each other's strengths and weaknesses, which has led to more interesting action within combat, and equally novel use of skills in non-combat moments.
It's a great system, and one you can get a taste of for free from Modiphius through the Quickstart booklet.