Man, I bet that's not a sentiment you hear often. Actually yeah, google searching that string doesn't turn up much. Neat.
For those unfamiliar, Vancian casting is the default means of spellcasting for editions between 1st and 3rd, where a spellcaster chooses his spells at the beginning of the day, picking both which spells he has prepared and how many of each, subject to some fairly tight limitations. Lots of folks hacked together their own "spell point" systems during this period, some of which were officially released during 2e, while 3e's Sorcerer class was the first official 'spontaneous' hybrid to my knowledge, which chose a small set of spells known and could then cast them interchangeably using slots rather than spell points. This achieved a flexibility somewhere between spell points and proper Vancian casting. 3e's psionics brought spell points back, and then in late 3.5 we saw a huge proliferation in alternate magic systems (stuff like incarnum and truenaming, which just ran on completely different mechanics from normal casting). 4e, for its part, did standardize things again, but to a new at-will / encounter / daily paradigm which bore little resemblance to anything preceding it, and which severely limited strategic flexibility.
ACKS, on the other hand, uses something very much like a back-port of 3e's sorcerer. Each mage has a spellbook, which contains the formulae for his spells, and also has a repertoire of spells which he has readily available for casting. Each day he receives a number of spell slots of each level, which he can use to cast any spell from his repertoire on the fly. The place where he beats the sorcerer is in his ability to change out the spells in his repertoire; while this is a slow and expensive process, it is a capability which the 3.0 sorcerer lacked entirely, and which is very limited for the 3.5 sorcerer.
There are a lot of things to like about the ACKS mage. It has a degree of tactical versatility, and also some strategic versatility, but the spells you learn when you level are (usually) acquired at random from other mages in the area. This means that getting the spells you want is hard, and puts mages on constant lookout for scrolls of spells they want. Better still, it provides a motivator for mage-vs-mage conflicts; to the victor go the spellbooks. It brings back something of the old Magic: the Gathering vibe, from back around Arena (sidenote - this is the best of the MtG novels I have ever read), where you duel for the rights to spells (which translated into the long-abandoned ante rules of the card game). The summoning spells in the Player's Companion really reinforce this feeling, and it pleases me. Likewise, the inability to choose spells on levelling means that mages aren't always picking the same handful of optimal spells; as I've mentioned, sometimes you're the evoker because you rolled all offense, and sometimes you're the dooromancer because you rolled Wizard Lock and Knock. Spell acquisition is its own game within the game, and casters are sometimes known by their own custom signature spells, which are not easily learned by others. It's really a pretty neat thing to have going on, and I think both of our mages are having fun with it.
The thing I miss about Vancian magic, though, is the planning and cunning that it entails. As a Vancian wizard, you have to make measured guesses about what you're going to encounter on this particular day, and then hope that you guessed right. An ACKS mage, on the other hand, must estimate how many more things he will encounter in a given day, in order to decide how to conserve his spell slots, but need not plan for particular expected threats to the same degree. I argue that having to plan in the Vancian manner causes players to think more like high-int magicians, whereas with ACKS-style spontaneous magic, mages are cautious due to their fragility but rarely play to the full capabilities of their nominal intellects. Roger has previously mentioned the "impulsive vs analytic" dynamic between fighters and wizards, and I think that Vancian casting reinforced that role for wizards. I recall from my time playing a Vancian wizard back in 3.0 that it was all about contingency planning. You have your combat spells, and stuff you might need at a moment's notice like Invisibility or Teleport, prepped with your actual slots. You have Scribe Scroll for a reason - scrolls are for all the stuff that you probably won't need today, but might reasonably need out of combat this adventure. Knock, Levitate, those sort of utility problem-solving capabilities. At later levels, you probably pick up Craft Wand, and shift some of your low-grade offense and frequently-used utility spells, like Detect Magic, out to those. If you're complaining about the ten-minute adventuring day as a 3.x Vancian wizard after 5th level, it's because you're managing your strategic resources poorly.
Now, you may remark, magic item creation in 3.x is terrible because it costs you XP. And this is true, and that is a bad rule, but I have a theory as to why it is there. I think, though I have no corroboration from any official source for this belief, that the motivation for magic items costing XP to create in 3.x is that it serves to slow down spellcaster levelling. It's a workaround for the uniform levelling curve introduced in 3rd edition, with the intent of restoring some degree of the old differences in levelling rate for classes which were clearly different in power. Because in Core 3.0 at least, playing a good wizard meant either scrolling it up or complaining about the ten-minute adventuring day. Sadly it would seem that the complainers have won out.
In any case, we do not see this same push to magic item creation in ACKS. Our wizards have not been creating potions or scrolls of utility spells with the same frequency observed in casters of similar levels in 3.0, and I believe this is due in part to the fact that they do not have to choose between fly and fireball, or invisibility and knock, in the same preparatory way. Created charged magic items are now useful because they permit you to use more slots in a day than you would otherwise be able, or with potions they let your fellow party members use actions to generate spell effects, which can confer an action-economic advantage. But the opportunity cost to create these items is quite steep; you need monster parts and many weeks and many gold pieces and a lab. Our mages have certainly not been jumping on the magic item creation bandwagon. Part of this, I suspect, may be that they just recently acquired the capability at 5th level, rather than 1st as would happen in 3.x. Perhaps I should just give it more time...
In any case the point was that playing Vancian casters generated a push towards gathering information about what you were going to face, and planning cleverly, which spontaneous casting does not to the same degree. And in retrospect, it was kind of fun sometimes, especially when you do have scrolls and other charged magic items as your strategic flexibility reserve.