I installed the game yesterday and have only managed to play four hours so far, but I would like to see people's reaction to the revised siege mechanics.  
 
I feel they are a huge improvement over previous games in the series. They ease with which units can be moved within a fortress and ending the dependence on siege equipment makes them much faster to resolve and can play out in more surprising ways. In the previous entries it was possible to predict the outcome of most sieges simply by comparing the unit types and numbers; the battle would usually boil down to two and immovable object rubbing against a very slightly movable object until everybody is dead.  
 
In Shogun 2 I was able to faint retreats within my own walls before smashing the invaders with a counter-attack, move arches to face the oncoming horde before pulling them back at the last possible moment. When attacking I was able to hold the defenders down on one side with disposable spearmen while moving my samurai to the other side of the fort to charge, surround and overcome their superior numbers. The tweaked mechanics, AI and level design allow me to be proactive rather than reactive. 
 
So what do you think, do my impressions tally with your own? 

In all, I prefer the siege mechanics in this game, but not for the reasons you specify.  I never had any difficulty winning in previous Total Wars, save in well-defended fortresses that could blow up my siege engines.  Obviously that's no longer a concern here.  I haven't had quite your luck with attacking.  It seems like any fortress with enough troops to defend every approach is virtually impregnable.. except versus a whole lot of archers.  With so few places to take cover in the castles, archers are beastly even on attack, whereas in previous games, they were virtually useless when it came to taking walls.  Maybe if I actually built siege units, attacking by way of the gates would be a more effective strategy in a major battle.
 
The best improvement about this game is that there are multiple capture points, which makes it impossible to just focus your defense in one place.   This makes it better strategically, but I think that the old way, of having a last bastion that you have to take, was probably more accurate.  The only problem being that there was no 'surrender' mechanic for demoralized troops with no general.  I mean really, why do they win the castle if they only manage to control half of it?   
 
Anyway, I've had no problem with assaulting castles really, I can do it when I need to, and when I don't, the wait time for sieges are much more reasonable.