Among the sequences (Possible spoilers ...):
The 12 plays back the already known Forli. In my opinion, the missions seem very fake. One constantly has the feeling that it has been attempted here to draw the season in the length. It happens not something exciting even something interesting and after max. an hour is over the magic. Somehow a pity AC II was really top story-up to this part.
The 13 plays in Florence. This time you get a new district to and lots of challenging tasks. The tasks vary from unnoticed assassinations, car chases and a flight mission. This sequence is very varied and some darn difficult. This is probably because that one has a lot of missions where you have to go unnoticed, as you would not know from AC II before. Mostly you have to closely monitor how the patrol routes of various squads run and then jump at the right moment to the next meeting. That made me very reminiscent of Commandos or Splinter Cell, which I thought was great. The story is this time even pushed a bit. Overall, you should therefore be concerned about 2-3 hours.
The bonus content is very bleak in general, and "gray" held. No comparison to the sunny and lively Florence or Venice from the main game. Gamerscore and Trophies are available for the end of the two sequences do not, which is kind of a shame. The Templars hiding in the style of Assassin tombs, are also quite nice, and provide more hours of gameplay.
Is it worth buying the GOTY edition or is the "standard" version? Hmm, no idea. Assassin`s Creed II is a top game and costs scarce 20. The bonus content are "quite nice" and cost 10 more scarce. For fans who want to know absolutely what happened in the missing sequences and not get enough, the GOTY version is certainly a great thing. For all others, it is unnecessary bells and whistles, especially since the action is virtually non-driven.