I picked this gem up a few days ago, and I'm having a lot of success, albeit somewhat mixed success.
Story of myself going from the controller of the County of Dublin to the Kingdoms of Ireland, Scotland, and England, and then controller of nothing spoilered for length.
The best kingdom I had started in Ireland (I always start in Ireland now). I started as Dublin, and dear old dad kicked the bucket less than a year in, so I gained control of his County as well. With two counties and roughly 250 gold I got when my father died, I created a Duchy and took another County. I revoked a title and now controlled all 3 County. As soon as I got the gold, I created the next Duchy and took that capital, revoking a title. I controlled 4 Counties. In 1070 I falsified a claim for Ulster and took over, and a couple years later I falsified another claim, took over, and made the Duchy of Ulster. I then quickly took over the remaining 2 Counties. With 3 Duchies and 7 Counties under my belt, and falsification plans in for another County which I would make a Duchy out of, I proclaimed myself de jure King of Ireland. By 1087 I was de facto King of Ireland. I ruled everything. A lot of my cities and such were starting to revolt due to my massive demesnes overcap, so I kept 2 Duchies for myself (The 4 Duchy and the 2 Duchy with Dublin) and gave away the other 3 Duchies to family members. My Heir got the 3 Duchy in the south of Ireland. I started falsifying claims so I could take over the Isle Duchy to give myself a grounds to take over Scotland. My original ruler died, and my Heir took over. After 15 years of waiting for a falsified claim, I said screw it, and built relations with the pope. I invaded the Duchy of the Isles or whatever, and took over. Scotland was upset because the Duchy of the Isles had a couple of his Counties, and so he claimed them and attacked me. I beat him off, and counter attacked, wiping out his 5k stack with 8k stack of Vassal Levies and a 6k stack of my own personal Levies. Peace settled down, and I requested an invasion on Scotland. Pope allowed it, and as soon as the treaty was up I took over all of Scotland (except the veeeery northern County.... Norway had it. Norway was HUGE). I had been upgrading my 6 castles 24/7 and didn't even have to call on my Vassals for Levies, I just pulled out a 11k stack of my own troops and curb stomped Scotland. After I had Scotland down, I decided to join some crusades.... but then I noticed that one of my kinsman had claims on almost half of England, due to a marriage I made and completely forgot about (daughter married a prince, and I made sure that the kids were mine). England was weakened from fighting Wales... cause the French were helping Wales out. I pushed claims for my Kinsman, figuring I'd just revoke and give someone the lands he claimed, and claim King of England for myself. King of Ireland, Scotland, and England. Fun eh? I beat England and my kinsman got the Claims, and I revoked. Vassals were PISSED at me, and I was barely containing rebellion. All of the sudden, my king dies. Moves to the Heir, and I have rebellion alerts everywhere. Apparently my Heir was an asshole to people, and nobody liked him. A few single county rebellions pop up in England and I squish them, and take the titles back for myself, further infuriating my vassals as my demesnes size is too high. When I finish fabricating enough claims and take them over so I can make Duchies to take over England and usurp the King, as SOON as I become king of 3 nations, half of Ireland and half of Scotland make a joint rebellion against me. I have almost no cash, and no Levies from vassals. I pull out my own Levies to try to take back at least Ireland, and it goes pretty well... until the French decide to show up on my coast helping my Uncle. I don't have enough troops of my own because England still suffers penalties due to being taken over, and I don't have any money to hire mercenaries. To make matters worse, my Heir dies from illness and my next Heir has 4 of the seven deadly sins, and horrible scores. To sum this up, the Kingdom of Ireland split into 6 different Duchies which fought over the scraps in the end.
Anyway, this game is extremely fun, but I have a couple questions. Most important question I have is this: How the hell do you continue to take over new lands without making your vassals hate you and rebel due to revoking titles and/or too many demesnes/duchies? If my vassals would just rebel one or two at a time I could curbstomp them, but when 5 of them rebel and my 2 main duchys are being taken over by roughly 10k-20k troops, I can't really fight back.