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they are worth buying, but i found them to be kinda short. took me 4 days to finish Revelations. so i played multiplayer till i hit prestiege.
if you want to follow the story of Ezio then yeah, get them.
You should definitely buy Brotherhood, as for Revelations that's kinda iffy. I don't know what's with it but I didn't like it as much as the other AC games.
I think you should get both just to complete the collection. Who wants to have an incomplete collection?
After AC2 I called it quits on the series, don't think I will keep going with the series anymore.