The first one is free of the DRM sillyness at least, not sure about the others.
Rather liked the first one, though the complaints about repetitiveness have some merit. Basically you go to a city, meet a contact who gives you some info on where to go next and then objectives unlock around the city. It's stuff like pickpocketing a target, overhearing a conversation, beating some info out of someone, etc. Once you complete a few of those you go back to the contact and the assassination is unlocked.
In addition to that, there's random things as well. For example, saving civilians being hassled by town guards. I liked the combat of the game, so I went roaming around town looking for things like that. Hunting Templars was good fun too: those guys knew how to fight. So much so I often handled it like an assassination, trying to get him from behind. I also managed to get my story-line targets in a "proper" manner for the most part, without it desolving into a lengthy sword fight.
2nd game is a lot tighter on the story telling, sending you all over town for the next cutscene, the next thing to do. The combat is a joke though; in the first one they could do a "guard breaker" move, so they could break through your block. No such thing in 2, just keep on blocking and you're pretty much invulnerable. The timing required to perform a counter is greatly relaxed as well: in 1 you had to time it to the moment the strike connected, in 2 you can do your parry as soon as you see them starting their attack. I don't think they ever countered my grabs/throws in 2 either, while those often resulted with me flat on my arse in 1.
I ended up fighting bare-handed all the time, disarming them and throwing their weapon behind me. The AI always tries to get it back.
Right, anyways I started making up house rules to make it all a bit more challenging, but it's just not there. Difficulty in 2 results from throwing larger numbers at you, rather than giving the enemies better moves like in 1.
tl;dr: first game has some flaws but the cities look superb, each with it's own character, and the gameplay gets a bit repetitive. If you like it I don't think you'll mind too much, otherwise it's a pain. 2nd game tells it's story much better but
imho the gameplay is catered to the console crowd (simplified/dumbed down).
Not familiar with the others, bailed on Ubi and their DRM, and like I mentioned I felt they were dumbing the series down.