The only major improvement I've noticed in siege AI between RTW and M2TW is the pathfinding inside cities. It's much better, at least with normal-sized armies. That used to drive me nuts in RTW. It's still not perfect... sometimes a unit outside the walls that's ordered to a point inside the city, won't take the nearest open gate or wall breach to get there, but run waaaaay around the outside to a different gate. But overall that aspect is much better. You can focus a bit more on tactics instead of micro-managing just about every step a unit takes inside a settlement.

Unfortunately the combat AI in sieges doesn't show any major improvement. The AI still does dumb, illogical things, especially on defense. I just took a settlement in my Danes game, where I breached the wooden walls at two different sides of a settlement with ballistas, then moved my army inside. Instead of trying to hold my two attacking forces at the wall, where they'd get some help from wall towers, they just ran to the town square and milled around, where they're easy targets for my artillery and archers.

The only way to have an enjoyable siege defense or attack, is to do it with an under-strength army relative to the enemy, and use house rules to avoid the cheesier exploits... like mowing down the defenders in the town square with artillery or missile units, or sallying and luring the enemy in range of your towers.