Well I haven't played it personally, but it is a whole new campaign with new factions and units, all for $2.50, so I'd say that was a pretty good deal. As for Fall of the Samurai, again haven't played it, but it's basically it's own game, and it's half off.
This is all a whole lot of strategy-ing, however, so I guess it really depends on if you think you'll play it enough for it to be worth $18. On that note it looks like you can get Shogun 2 + Rise of the Samurai + Fall of the Samurai for only $20, which I'd say is an even better deal. If I wasn't already looking at other games (and also taking a summer class
) I'd bite that bullet pretty hard.