I'm a big fan of AU. Started running a home campaign about a month ago and the players are having a blast.
Akashics - I like the idea and feel of the Akashics, but consider them difficult to work into a non AU campaign. It's definately the most setting specific class in AU aside from the racial ones. The various akashic abilities are flavorful and lead to some hard choices as one levels. As a DM I'm having a bit of trouble deciding exactly where those abilities are coming from, but I'm not too worried about it just yet. The Akashic in my party is a Mohj, and is having a lot of fun. He hangs back in combat and attacks with a bow, and really shines when it comes to problem solving or ferreting out secrets.
Champions - A good example of what Paladins could be. Don't get me wrong, I like the Paladin, and the 3E paladin is probably more powerful than a Champion, but the Champion is far more versitile a class that can be used for a great many types of characters. I really like the cause specific powers as well as the Call Upon the Power abilities. I really don't like the summoned shield and weapon. Each one is only usuable once per day for a short period of time. Eh. I'd much rather they be able to empower thier /existing/ shield or weapon. The summoned armaments would be very nice in certain specific situations, but in general it's better to just have a shield on your back and a sword at your side.
Greenbonds - Eh, not my favorite class. They're definately effective and versitile, but when I read them, nothing sparks interest. They're a cleric/druid hybrid with some nifty abilities. The Greenbond in my party is enjoying the class and that's good enough for me.
Mage Blades - I /love/ the Mage Blade. Best fighter/wizard class I've ever read. It blends the two aspects nicely, leaving a class that is flavorful, effective in a fight, and balanced. I do /not/ believe a multiclassed fighter/wizard has the same feel. 3E made Fighter/Wizard a suboptimal choice, and that's putting it kindly. While Bladesinger is a nice prestige class, I think the concept is prevalent enough in fantasy to deserve a core class, not some prestige class it takes a sizable chunk of a campaign to qualify for.
Magisters - Nifty wizard variant. They are the /best/ spellcasters in AU bar none, and it really shows. 9th level spells with all simple and complex makes a big difference. The Magister in my campaign loves the versitility he has in AU. Of course, that's more of a comment on the magic system than the class. The class is alright, nothing too special, but the magic system is so good that this class really sings.
Oathsworn - I really enjoyed the Chronicles of Thomas Covenant. Oathsworn /are/ the Bloodguard. Low level Bloodguard, but bloodguard never the less. I also think they're one of the weakest classes in the game. Their abilities are flavorful and eventually very powerful, but at low levels they just can't equal the other melee classes. Now, even with that criticism, I like the Oathsworn. It's a class you take because you want to play it and want to be intimately woven into a story. You don't play the class to be UBER. You play it for the story, and I'm fine with that. The Oathsworn in my game is having a good time. He doesn't dish the damage the Warmain does, but he holds his own and really enjoys playing the character.
Runethanes - Another class that starts out weak and ends up scary. It's an investment class. You'll be well below the power curve until your class abilities start kicking, but at that point you're a force to be reckoned with. The first major NPC they met was a High level crippled Runethane, who powered them up before sending them goblin hunting. He impressed the heck out of them with his runes, and I was able to show the group some of the high fantasy aspects of AU without giving them permanent items.
Totem Warrior - They make wonderful NPCs. As a DM, I can use Totem Warriors for instantly themed NPCs that the party can understand and relate too easily. The class archtype doesn't really do anything for meas a player, because I'd rather play a Mage Blade or Unfettered for my melee goodness, but they are still an extremely sound class. The animal companion can be an incredible boon for the party, and the totem powers + Bonus feats make for an effective character. I also really like how the one class has a number of different variants making for easy customization.
Unfettered - Swashbuckling goodness. A lightly armored fighter that can hang with the big boys. Now, don't get me wrong, this is a finesse character. Warmains have an easier time dishing out big damage, and have a higher armor class on average, but then, that's the bar they're measured by. Unfettered have better mobility, probably a better armor class against that one particular big baddy, and are sound in a fight. With teamwork, they can get off a fair bit of sneak attack, adding to thier damage output. The Unfettered have a few advantages over a Fighter/Rogue. Better BAB, Better HPs, a free Level based AC adjustment, and some spiffy class specific abilities. A Fighter/Rogue by contrast has access to better Sneak Attack, better skills, and more feats. It's a tradeoff, but I'd rather have an Unfettered. It's already tailored to my desires.
Warmain - Classic Big Tank Fighter. I prefer them to the fighter because of thier focused role. The 3E fighter is as general as possible. You turn it into what you want. The Unfettered and the Warmain split the Fighter into two classic archtypes. The Tank and the Swashbuckler. I think the two concepts are different enough to merit thier own treatment. YMMV. Now, the Warmain is /the/ combat monster of AU. He's huge, tough, hard to hit, and strikes for a ton. The Warmain in my party is (Surprise, Surprise) taking Giant racial levels as well and has been extremely combat effective. Now, he's also largely pacifistic and dislikes entering Chi-Julud, so that's toned him down a bit. He loves the class, and I think it's just fine.
Witch - At first I read witch and went, eh... Second tier spellcaster with spelllike abilities for flavor. Doesn't seem worth the trade. Then, upon second reading I sat down and thought about some of thier abilities. Witches are mighty cool. They aren't the masters of the Arcane the Magisters are, but thier spell like abilities can be extremely potent. The Wood Witch is probably the best of the bunch for party cohesion. Healing combined with a nasty ranged attack vs. undead. The hardest thing about most of the witches is actually selecting which witcheries to select.
I think the classes in AU were well done. They aren't perfect, but my quibbles are small. The Champion needs some Tweaking to my eye. The Oathsworn could use a little help. I'd like a non mystical Skill monkey. Beyond that, I'm very pleased and have enjoyed my new AU campaign more than my last 4 D&D games.
On a final note, my opinions are also modified by my general dislike of Prestige Classes. 3E seems to be developing a philosophy of 'The core classes are just what you take to qualify for the prestige class that /really/ defines your character.' I'm unhappy with that feeling. I'm also unhappy with the rampant power inflation of Prestige classes. I feel that the core classes should be interesting enough to stick with for the duration of a character's carreer. The stock D&D classes did a poor job of that. 3.5 is a bit better as far as the core classes, but then they've fully adopted the more powerful prestige classes as thier standard. That just doesn't sit well with my style of play.