I've only played the first one. It's a fun game, but it was very repetitive. If I recall correctly, there were three main cities that you could go to, and nine assassination targets. So you played in each city three times, as opposed to having new levels for each mission. Nothing new happened for each mission, other than the cities adding more guards due to your previous assassinations there.
The sequels were supposedly better, but I never got around to playing them.
The only thing I've heard about the third one is that they made it way too complex and large. One reviewer said he played the game for five hours and was still receiving basic tutorials. It's gotten solid reviews, but not the highest.