Invisible Sun is a mad game about wonder workers in a surreal world behind, and superior to, the grey world we live in. It does this brilliantly in many many ways, though there are certainly places its lofty goals haven't quite been reached.
I'm going to talk about a couple different parts that really make an impression on me, good or bad: Creating characters, The World(s) of the game, the Rules and Actually Playing the game. I should also quickly mention, as others have, the Ambiance of the game is phenomenal. The art is gorgeous, the cards are perhaps a bit overwhelming but well made and attractive, the Black Cube is a freaking magical way of transporting/delivering you the game (albeit a HEAVY magic).
Character creation is Fun, which is a requirement for almost any game to be good in my eyes. Everyone is a magic-wielder and playing this game will make you hesitant to use paltry words like Mage or Wizard to describe them because they just don't do it justice here. The Classes/Schools/Guilds are interesting (except for Vances, that feel like the class to play if you just can't wrap your head around what this game is capable of)) and it makes deciding what to play a lot of fun, which is good because the rules aren't always super clear about the various bit mean (and there is a lot of new vocabulary in here to absorb).
Characters start pretty deep, which is wonderful. You not only create your character but work on defining/describing the people you know or who know you and why, Everyone starts with the strong possibility of having a house and income, a pretty shocking idea for most games, and there is a great mini storytelling session just deciding who your neighbors are! This provides a rich opening to adventures RIGHT out the gate. You can easily build a character and have 4 or 5 things YOU want to dig deeper into your character and where they live, day 1.
Characters are not defined by skills, which was a shock to me and took some getting used to (I only have three skills?!?) and is usually quickly remedied in-game if that remains important to you, which is might not. You have deeper things to understand than simply Skills.
The World(s) of the Actuality are well done at a theoretical level, but actual play has made me reach back to my Planescape days for deeper inspiration. The worlds of Invisible Sun are surreal and interesting but, perhaps intentionally, so vague as to demand that you fill in a tremendous number of blanks yourself. Again; this can easily be seen as a Feature, not a Bug. I'm perfectly happy to build them in the image my mind sees fit and believe the game all the richer for that lack of author-mandated definition. I DO believe that some GMs would be looking for more here, so in my mind this is a count against IS, but if you love inventing insane worlds like I do, you won't miss this much.
The Rules. Are kind of a beautiful mess. There are rules we didn't realize existed until after months of play. Rules we have decided as a group to ignore. The team did a great job of annotating "See page x in Book y" but there is simply a LOT here. Getting the PDF version so you can text search through it is strongly suggested.
I meant what I said about beautiful mess though, it has some Great stuff in this game! My favorite rules are How Characters Earn XP, How characters earn Despair and Joy and how you learn new things, the concept of Secrets, how you increase standing in your organization... some really really good stuff in there.
Actually Playing the Game. The ultimate test of any game is actually playing it, and while my group and I have certainly spent some time bemoaning various hidden rules or undefined things, that pales in importance because the Game is Awesome.
Within six adventures we were playing characters whose lives were worthy of any epic saga. Rich emotional situations and intractable problems, complex needs or desires and a built in group of friends to help you through them all. This isn't accidental, this is what Invisible Sun is really aiming for and holy cats they hit that goal straight on and full impulse. The game system of Joy and Despair MAKES characters intentionally choose or set up negative outcomes within their own personal stories and this is critical for their advancement. Having a system where characters get a needed reward for NOT succeeding at everything makes characters much more real and deep and interesting. Examples from my campaign:
"I tried to understand why my character's Father was undermining all my successes and trying to ruin my standing and... well... I found out, but this knowledge just made everything worse. I have reason to believe I'm an illusion or a changeling or a spell gone wrong, that I don't even really exist!"
"My character and her girlfriend had an instant bond, like we'd known each other for ages. The more we learn about each other the more we learn we MAY have actually known the other before and our mutual forgetting may have been because something terrible happened. Do I even WANT to find out?"
These are pretty dark experiences for a character in game and where in other games this would entirely be the result of someone deciding this is what they want and the GM saying "Well, I guess we can have other people sitting around while you roleplay out your soap opera" in this game it is organic to the game and I cannot fully explain to you have fabulous the results are.
Another fascinating mechanic is one I have heard people use in other games but in this game really shines. The game comes with a "Sooth Deck", a tarot-like deck of cards with suggested meanings. Sure, you could just roll dice and get a Success or Failure but many situations are better served by having players state what they are trying to accomplish and drawing a card and riffing off of what is shows.
"How did the big party I threw work out?", "I am going to ask the Spiders if they know anything about this Death Cult", "I have this strange orb of wild magic from a couple adventure ago. I'm out of ideas so I guess I'll just see what it does." None of these are clearly "make a roll against X" and pulling from the Sooth Deck gives you some really great ways of figuring out what could have happened.
The last thing I'll touch on is particularly unique to Invisible Sun: Time and Reality bend and stretch here unlike in any other game. You want to rebuild a character? Any time you want. You made a ruling you decide later is bad? The world has changed JUST NOW and that's just how it is now. The world is ever changing and changeable to a degree that few others can even conceive and you can weave stories here that would be impossible in other systems. You want to create a monster that eats familial love? Done. Want a form of succubus/incubus that doesn't seduce you but rather goes back in your timeline and replaces someone you love with themselves? Done.
I'd like to wrap up by saying how very impressed I am by the feeling of this game and the key mechanics it uses. The rules have some issues and balance is not always well addressed, but the game itself BLAZES. Every game we play digs us deeper into the stories about these characters in ways you don't often see until near the end of a long campaign in other systems.
Full disclosure, I’m a member of the MCG Asset Team, who demos games at stores and conventions and could receive swag through that program. I'm not getting anything for this, this is my honest opinion. This is the only game I have done this for, because I liked the game so much. I've run a couple games at a local convention to great success and am happy to tell you what I think, just ask and I'll expound!
TL;DR: The rules are a bit of a mess but game play makes some of the deepest, richest characters we have ever played and we're having a great campaign. One of the best storytelling games I've ever played.