I own all the Total war games. I have a big problem with Shogun 2; apart from different AI (and I say 'different' not 'better' for a reason, see below) and updated graphics (plus a few minor other touches), there's not much I haven't seen before.
There's all sorts of really groundbreaking things they could have added, such as a real time campaign map (have a look at Defcon to see how this might work), or the ability to have a 'parallel universe' mode where you can rewrite history by developing technologies that were never developed (but would have been feasible at the time, such militarised hot air balloons or 18th century versions of the Maginot line and other such low, 'anti cannon' structures to replace forts... not fantasy stuff, but stuff that could easily be used without destroying the 'sense of the period').
What would have been **really** cool would have been a new 'War council' window, where you can plan high level tactics with your allies... things like agreeing which city to attack, so that you and your allies send armies to lay seige to the same city by the same turn (after the appropriate money has changed hands of course), or instructing a particular faction to protect your back as you move forward to attack an enemy, or simply asking a protectorate for some of their unique units (effectively allowing the trade of mercenary armies between nations, and so you can build multi-national armies... how cool would that be - attack an Ally and their mercenary units in your army turn against you; politics and strategy finally start to blend). NB - I can't see the War council idea being difficult to implement as this actually occurred in Medieval 2, where the Vatican issued crusades i.e. factions had to send an army to seige a city by a certain turn).
All not to be. Perhaps the worst disappointment is the AI..
Yet again with Shogun, the AI starts off looking great, but a few plays in in, you can beat it straight away. Despite the sales blurb, the AI isn't that great. I won't give the game away for Shogun 2, but the previous Empire AI certainly had a problem with cannons; it didn't understand line of sight (AI cannons spent all their time shooting at the sides of mountains in hilly maps) and the AI always sent cavalry on suicide runs against well defended cannons. The Empire AI was also useless in defence; you just (1) pick off the enemy horses with your cannons, which (2) leaves the enemy cannons defenseless, then (3) hit the enemy cannons with fast moving horses from the side/rear (which is now easy as the enemy has no horses of its own to counter), and finally, (4) decimate the enemy infantry with your cannons and/or move your infantry into buildings if the enemy start advancing. You could beat the Empire AI consistently not because of skill, but because of the known mistakes it made.
Unfortunately, it is looking like the Shogun 2 AI is the same, albeit with different mistakes.
I think the big issue with all Total War Battle AIs is that they never realise that combined (or even paired) units can have a weighting greater than the sum of the parts, and that the weighting of many units changes with range or location. The AI just cannot move its forces in a way that increases its multipliers and certainly not in the same way a human opponent would. Once you realise that failing, all total war AIs are simple to crack.
Another big issue I've always had with the Total War battle AI is that you always seem to be playing against the same opponent. It doesn't matter which historical general you are playing agaist because the AI general always plays the same strategy. In real life, Napolean fought one way (heavy reliance on cannons), whereas the Prussians fought a totally different battle (split army into two, one side holds the enemy to the front, whilst another line attacks from the side), whereas the British simply built a massive trading empire (complete with massive navy to protect the supply lines) and then funded everyone else for most of the time (hence 'Nation of shopkeepers'). I'm not that knowledgeable with Japanese tacticians, but unless all Japanese generals used exactly the same tactics on the battlefield, the Total War AI is still emulating the same single nameless general it always did.
For the campaign AI, it is still not true to life either. In real life you get power blocks building up, where *all* weaker nations become protectorates, and you cannot attack any weak country without incurring the wrath of a superpower. Such political dynamics is non existent in any of the Total war games (because in the game model political ties are too weak to promote such dynamics). As implied earlier with my 'War council' idea, political ties are also still not really useful - you cannot work with your allies in any tactical way (other than trade) as there is no real way to attack and defend together.
Finally, in the world of quad core (or i7 x8 threads) processing, it would be expected that the Creative Assembly would finally write multi-thread efficient code. They haven't.
Shogun 2 only uses about 10-15% more processing power than Napolean total war (and yes, I set the graphics way down to ensure the graphics card wasn't the bottleneck). Sure, the Shogum 2 engine uses 50-60% of a quad core, but that is only as efficient as other notable game engines were 2 years ago (and implies dual core optimisation). If you have an i7, then much of your extra threads will be wasted for now. Maybe ok for a mindless first person shooter, but you would expect better from a deep strategy game.
Given all this, Shogun is the second Total way game I wish I hadn't bought on release (the first being the original Empire Total war). My advice is to wait 6 months until the price comes down a little, the AI is a little less obvious (either via updates or third party AI patches), and they start throwing the DLCs into the deal.
The Total War games are still the best strategy games out there, but I won't be buying them on release again; this is worth 3 stars on release. but perhaps 5 stars in 6 months time.
*** Update June 2011 ***
The price has now halved from release, and the all important post release patch has just been released (lots of little fixes plus full DirectX 11 support). Now is probably the time to buy Shogun 2 if you haven't already.
Post patch, Shogun 2's AI still has an awful problem with understanding doors (which is really not the step forward from 'a problem with understanding cannons'), but hey, at least the factions are more confident than in Empire (they actually attack each other in Shogun 2, and know how to invade from the sea), and although there are fewer units and little difference between faction units or maps in Shogun 2, at least Shogun 2 is more strategic.
Downside is that the units in Shogun are all a bit similar, with very few classes, little variation between factions, and no variation in the maps (Japan is a country with little climatic variation, etc).
*** Update May 2012 ***
The price is now less than a quarter of the original asking price. It is a steal at this price!