I bought this game a few months back, am on my second grand campaign. While I've heard all the horror stories, I've experienced very few problems.
A couple of crashes that I cannot exclude PC components overheating/failing on
One instance of the bug where ships think sea is land that lasted one turn (or a reload, I can't remember which) but which is really irritating when they have that smug "That's land...sir..." and "this...is a ship, sir" which really winds me up anyway
I'm playing this campaign on hard/hard and the battle AI seems ok, some dodgy moments where you can force charging to units to stop in front of your muskets just by rearranging some of your forces etc but generally I have to fight quite hard in any battle that the meter gives as even or greater.
The campaign AI however is fairly shocking - the way you can leave Britain completely undefended because they'll never land on it, and other things, it just feels like there has been very little I in the AI for the campaign map, you can predict an army's next move with little bother, and they pillage the hell out of your towns and farms etc in ways that gain them absolutely nothing (that's fine if its native american tribes, but you'd think France or Spain might actually have plans behind their movements)
The diplomatic negotiation screen is generally pretty useless (at least as Britain).
The campaign interface is also fairly unhelpful unless I'm overlooking some features - when I have some spare cash there's no easy way to plan my building upgrades, I just have to scroll through the towns and see whats what (when there's 30+ that gets very tiresome). It is full of little niggles like 'you have not met the requirements for this tech' without actually telling you what they are etc
However, I am still enjoying it quite a lot,