Picked up a copy of ACKS in pdf this last week and have been slowly working my way through it when time permits. So far I'm very impressed. It feels like a compromise position between 3e and OSRIC, almost, with the underlying structure closer to OSRIC, but with player options and a number of simplifications reminiscent of 3.x. Ability scores, for example, are on a flat modifier system like 3e's. Given that we wasted probably an hour of chargen for OSRIC explaining (and re-explaining) the ability score charts, this is a Good Thing for me. But, the modifier categories are variable-width, with a narrower ranges of bonuses and penalties, much like Traveller. Also nice; a high score is now good, but less overwhelmingly important than it was in 3.x.
Proficiencies are also very much along the 3e vein; they're kind of a combination of feats and skills, and you slowly get more as you level. Since the core classes don't really have class features that increase with level (mostly), being able to differentiate yourself from other members of your class via proficiencies is quite nice. I also really like ACKS' handling of races. Their core only includes three - human, dwarf, and elf. Humans have access to a wide range of classes: fighter, cleric, mage, thief, assassin (kind of a fighter-thief blend; no more instant-death attack), bladedancer (dervishy cleric variant), explorer (wilderness fighter / old-school ranger), and bard (actually not terrible). Dwarf and elf have access to two specialized classes each; Dwarven Vaultguard (dwarf fighter), Dwarven Craftpriest (dwarf cleric), Elven Spellsword (elf mage/fighter), and Elven Nightblade (elf mage/thief). These classes roll the racial bonuses into the class itself, and increase the XP required to level accordingly. Demihuman level limits are back, but humans also cap at 14th in all classes, with dwarves and elves capping between 10 and 13 depending on the class. Also interesting is the vast reduction of minimum ability score requirements; dwarves require a minimum Con of 9, and elves require a minimum Int of 9. That's it. No more "Arg, I can't play a gnome because my Dex is 1 point too low" dilemmas during character generation. Each class also has one or more prime requisites, which also require a 9 or more, but these can be raised by sacrificing points from other scores, and you get bonus XP for having high prime reqs, just as in OSRIC, though there are tiers of XP bonus (+5% and +10%), and they're more standardized across classes.
Speaking of ability scores, ACKS advocates 3d6 in-order, which is harsh. To counterbalance it, though, they also suggest that each player generate five (5) characters, choosing one to be their main PC, and two as backup PCs for when the main dies. The final two are to be given to the DM (er, Judge in their parlance; I like the Judges' Guild homage) for use as NPCs, potential henchmen, rival adventurers, and similar. I quite like this, for a number of reasons. It can generate dilemma situations and interesting choices for the players without totally screwing them, but also generates free content for the DM. Five is a lot, though, especially if it's their first time building ACKS chars. Three might work too, with one main PC, one backup, and one NPC.
Speaking of backup PCs, they have a highly entertaining rule for generating XP for your next PC if you don't want to switch command to one of your henchmen, if you have any. And you probably should have henchmen; porters are less important than they were in OSRIC, since you can carry more, but Charisma is useful again, since it lets you have many and loyal helpers (and backup PCs for when you die). And you will probably die; save-or-die poisons are the norm, as in OSRIC (though some are save vs poison or be paralyzed, I believe). The Death and Dismemberment table is also quite good; reminds me strongly of d20 Warheart's. There's no level loss for resurrection, but you need a 7th level cleric, and you also have to make a roll on the Tampering with Mortality table, which can generate many and unpleasant side effects of going beyond the mortal coil.
For those adventurers who do make it to high level, though, awesome stuff awaits. There's a whole chapter (and a long chapter, at that) on fun stuff like spell research, magic item creation, building castles and cities, running thieves' guilds, ritual spells (stuff like wish), raising undead armies... pretty much everything I mentioned here as "stuff you should be able to do in high-level D&D", and then some (magical crossbreeding to make owlbears, for example). It's... pretty amazing. I've seen whole sourcebooks on some of these topics that did poorer jobs of it. Granted, those books were written for much more complex systems, but still... ACKS has done a very concise, but functional job.
The trouble is that these ventures take a lot of money. Fortunately, ACKS awards XP per gold piece extracted from the dungeon, and even suggests a standard ratio of XP from monsters to XP from gold that should be awarded. Thus, since you know how much XP a PC of nth level will have, you can use that ratio to determine about how much gold they should have earned over their careers. This makes starting out of levels higher than first pretty easy. ACKS also provides mercantile ventures / speculative trading rules reminiscent of Traveller's, but simpler and more accessible. You know those caravans that low-level PCs get stuck guarding all-too-often? Well, turns out they're owned by mid-level PCs trying to move silk at a profit.
Which brings us to the worldbuilding angle. ACKS has some very interesting things to say about fantasy worldbuilding on giant hexmaps, but most interesting to me is their advice on building a 'borderlands' region as a setting. It's like they went "Hey, you. We heard you wanted to run Western Marches, but are a little too lazy. Well, we were in the same position once, and here are all the things we think you should know." It's good stuff.
In short, I feel like ACKS accomplishes several of my goals for Fantasy Traveller (relative simplicity, old-schoolishness with some degree of differentiation between characters of the same class, hexmaps, lower power curve), while also nailing high-level play. I have a bunch of career tables queued up, though, so they're still going to come out of the pipe, despite the fact that they will likely not see use any time soon. ACKS is definitely on the to-run list, hopefully for this summer, and quite possibly into next fall semester as well.