Dave Hargrave's Arduin and Steve Perrin's "Perrin Conventions" were influential in creating the West Coast style for D&D back in the late 70s and early 80s. Both of these lead to a style of play, and a style of game worlds that were different from the "mainstream" D&D that TSR was selling. This West Coast style lead to The Arduin Adventure and also to Runequest. There are references to DMs and designers like Perrin and Greg Stafford throughout the Arduin Grimoires. Like Bigby and Tenser in the OD&D rules, there are spells and monsters named for both Perrin and Stafford.
So, what is the Arduin Adventure? It is a fantasy role-playing game, class and level-based, very much in the mold of OD&D. In fact, these codified rules owe a lot to the Holmes boxed set for D&D that helped to codify and mold those rules into something cleaner and clearer. The influence of Holmes' rules, however, are filtered through the imagination of David Hargrave.
If D&D is Tolkien's Middle Earth and Moorcock's Young Kingdoms as filtered through the imagination of Gygax and Arneson, then Arduin takes those influences and a number of science fantasy ones (Star Wars being very important to Hargrave) and pulls them through the psychedelic experience that was the mind of Dave Hargrave. I consider this to be very much a plus because Arduin definitely has a much stronger voice to it than D&D did at the time, perhaps because it was the vision of one person instead of a growing committee. Don't get me wrong, D&D is a great game (I play one of the retroclones of OD&D on a regular basis), but it does not have the voice to it that Arduin has. For some this might be considered a weakness, but I think that it was a strength of the game. I believe that Arduin was the first RPG that was as much the vision of its author, rather than just a way to come up with some rules that could be used within a certain genre. I think in this way, Arduin is the spiritual father of games like Kevin Siembieda's Palladium Fantasy and Rifts. There are a lot of similarities between Arduin and Palladium Fantasy to me (but that is probably something for another post).
Now, while The Arduin Adventure has everything that you need to play, it is really not a complete game. Much like how the Holmes version of OD&D covers only the first three levels of play, so does The Arduin Adventure really only cover the equivalent for Arduin. You have enough to get play started, and play for a bit before having to "upgrade" to a fuller version of the rules in order to continue. If you have The Arduin Adventure and the first three (at least!) Arduin Grimoires you can fill in a lot of the gaps and play for a while. If you're interested, Emperor's Choice does offer a print version of The Arduin Trilogy that contains the first three of the Arduin Grimoires and The Arduin Adventure. This thick book will give you a lot of gaming, whether you use Arduin's native rules, or plug them in to D&D or some retroclone of it. If The Arduin Adventure whets your appetite for Arduin, then I really recommend getting the Trilogy in print.
All of the things that are familiar to OD&D players will make Arduin easy to pick up. The classes are basically the same, races are handled in a similar manner, and spells and advancement are very similar. Moving between the two games would be ridiculously easy. Picking up Arduin will not be difficult, if you already have a familiarity with OD&D or various OSR games that duplicate the experience of it. If you aren't already familiar with the "old school" approach to fantasy games, Arduin may cause some problems for you because it does assume a familiarity of that style of play. However, at 66 pages, reading Arduin and picking up the rules shouldn't be that much of an investment of your time. Whether you want to pick up Arduin in order to learn about an old school game that you may not have known about previously, or if you want to pick it up because you want to supplement your OD&D/OSR games with some new material, I think that you should be picking up The Arduin Adventure (and then moving on to as many other of the Arduin Grimoires that you can find). You will not be disappointed.
Combat is a bit more complicated in Arduin than in OD&D. Dave Hargrave liked his critical hit and fumble charts. Each weapon hits differently, depending upon the Armor Class of a character's opponent. This looks more complicated, but the combat tables in The Arduin Adventure and The Arduin Grimoires make this process much simpler than it should be. Regardless, this is still OD&D at its heart, and that game really only gets so complex. Some may see this added complexity as a boon in their old school games.
On the negative side, this is an ugly PDF, however as it is a reproduction of the original game (layout warts and all), I don't know that I can hold that too much against the publisher. Much like with the older edition D&D PDFs available, this book is an artifact, a reflection of its time. If pretty and shiny is a requirement of your role-playing games, then The Arduin Adventure may not be for you. However if you like rough and tumble RPGs that do at the table what they say they will, you will want to pick this up for your gaming library. And then, go to the Emperor's Choice website and buy more Arduin stuff.
All in all, if you like old school style D&D and you haven't already experienced Arduin, you should buy The Arduin Adventure and kick the tires. I think that you will like what it can bring to your gaming table, even if it is as a supplement to your OD&D/OSR game of choice. Go out now and get your copy.
[You can find a complete review over at the Dorkland! blog: http://dorkland.blogspot.com/2013/08/the-arduin-adventure.html]