I want to take 13th Age for a spin among my friends, but I don't want to buy the books before trying it out for a few solid sessions. The 13th Age SRD is available freely online. Can we use that to get the full 13th Age experience, and if not, what exactly will we miss?
13th Age SRD is sufficient to enjoy benefits of Archmage Engine.
You will be missing out however. For example:
Rob Heinsoo and Jonathan Tweet's writing. In the introduction they promise to provide a product of their long dalliance with RPG games. Rules are provided in friendly informal and entertaining way: "We aren’t telling you exactly what..." to do, but you will get default option and hints/advice on variants.
Core setting and icons. They are presented in a way to give you room for interpretation and to grant you opportunities to do stuff. While a bit irksome to old school fans like me (I do love hard facts), the players are likely to enjoy being the center of attention, and the world just waiting to follow-up with crazy shenanigans.
Speaking of icons, without core book, they are unlikely to be useful. Consider skipping icon rules until you get the core book.
13th True Ways provides very advanced classes and complicated monsters. Bestiary was probably the most awesome book of monsters until Bestiary 2 was released.
The list would not be complete without mentioning living dungeons and Eyes of the Stone Thief. Living dungeon is a malignant entity that thrives on obtaining new buildings (rooms), and treasures, and creatures. Stone Thief is a Moby-Dick of living dungeons. And Eyes of the Stone Thief is probably the most incredible campaign built around hunt for an elusive predator that ate your home town!
That said it is strongly advisable to review/obtain the following free stuff:
13th Age SRD PDF (obtainable from DriveThruRPG) is the most up-to-date and complete version SRD. It also contains quite a few very important rule update and clarifications. I consider this a mandatory version of the rules.
Form fillable PDF of character sheet. Printer friendly.
Master list of online resources. Make sure that your players read one-pager summary of the system, while you, a GM, keep two-page system core at your fingertips.
The SRD is a great reference, but the book itself is a joy to read and I'd very strongly recommend getting a copy. It's not just that you are missing a very attractive layout and a highly readable style, but you are missing quite a bit of world detail: monsters, description of cities, monsters and the icons, and so on.
You are also missing the game master asides and hints by the authors, which are interesting AND informative!
Here's a random example (on background points):
Through most of our playtesting, we used varying numbers of background points for each of the classes. We were sticking with traditional fantasy interpretations of the rogue as a high-skill character and the fighter as a low-skill character. We never felt entirely comfortable about this traditionalism. We included equal background points an optional rule. But our playtesters had stronger feelings. They hated giving some classes fewer background points. They felt it betrayed the game’s storytelling possibilities. So we finally decided to write the rules the way people want to play instead of acting like cantankerous traditionalists. So if you are a GM who wants classes to start with unequal backgrounds (and the class talents that help the rogue, ranger, bard, and wizard in certain directions aren’t enough for you), use our original background points numbers as your optional rule: barbarian 6; bard 10; cleric 8; fighter 6; paladin 6; ranger 10; rogue 10; sorcerer 6; wizard 8.
For me, as a GM these direct messages explaining things are very helpful -- they make me understand when something is debatable, present other options, and explain the "tone" of decisions. Plus, fun to read.
For me, $25 for the PDF is an easy choice -- it's a single book with all you need, beautifully written and (yay!) well indexed. I think your main decision will be if you want the hard copy also!
You can play it fully (especially if you plan to use your own icons instead of the suggested default), but the writing in the 13th Age books is some of the best in any RPG books ever, and you're missing out on a ton of hooks, suggestions, and flavor.
The default icons and the default setting aren't fully fleshed out (intentionally!). I say at least the GM should get the core book for the flavor and hooks.
Mechanically you'd be fine, all the classes and their abilities are fully fleshed out on the SRD, and most if not all of the combat, skill rules and class creation rules are there. You'll miss out on pretty much all of the flavour, including the icons details, missing the bestiary obviously, and pretty much all of the Writer's Tips and tricks in comments.
The SRD will provide about 99% of what you need for the full 13th Age experience. The most up-to-date SRD (13 True Ways classes), a PDF of the world map, and a level 8 Organized Play campaign are all free on DriveThruRPG.
Organized Play is a free resource Pelgrane Press/Archmage provides that sets up all the work for a GM to run a campaign for even levels (i.e. 2, 4, 6, 8). They provide GM tips, explain montages, have monster stat blocks built into the PDF story, and also list magic items.
There are additional free resources from the Pelgrane Press website that are also helpful. My recommendations here are the pre-made characters / blank character sheets and the power decks (if players want printable reference cards for the spells/skills).
You can also get more free Organized Play campaigns at the same site. These will give you plenty of opportunities to try the system out for a few solid sessions.
The biggest "cool factor" of the 13th Age experience, if I had to pick one, is the Escalation Die. This is on page 331 of the SRD, and is a great tool that can be used for any RPG system. It is a small/easy feature, but it empowers the players as fights drag on, and helps the GM by preventing combat from taking up a bigger chunk of your game time.