From what I played (first 3 assassinations), the targets are set up differently (which has an impact on the strategies you can employ to approach them) but what you do in the end is up to you.
After the main kills there has always been a huge chase for me (but those were half of the fun — the controls, the free-running / scaling mechanic all comes together in these moments). But some informants also asked me to perform proper stealth kills (which aren't totally easy).
As for difficulty, I find it about right. I died about 4-5 times so far, but the last checkpoint is never too far back (as they seem to occur after you've either travelled a long distance or achieved something).
And climbing up a rooftop without breaking a sweat and walking towards an archer guarding up there, him warning you that you have no business here, then you breaking into a sprint and suddenly lurching at his throat and killing him with the hidden blade before he can alarm others = fun!
I found out that the "collect the flags" missions from the informants actually form are a proper route for a race through the city, which — with the free running — is properly awesome!
It's like a city checkpoint racing game!
Re: Comments on stupid AI
The things about the "stupid AI" is that it works by clearly defined rules (which are conveyed to you early on). That's more a question of game design than incompetent AI programming.
Yes, it leads to perceived stupid behaviour, but it also means that you know at all times what you can get away with, and what you can't. So I think that the way the AI works in this game is definitely not to its detriment.
The rooftop gardens are a game design choice. Yes, it is somewhat unbelievable but it's got nothing to do with "stupid AI". As an aside, they only work if you enter one unseen, e.g. if you already have plenty of guards on the rooftops, it becomes rather difficult to slip into one unnoticed.
