To be honest, I liked the original better than the sequel. The original, in my opinion, has better quests and characters (aside from Kreia). The only thing the sequel does better was that a) one is able to start as a Jedi, and b) there is the 'influence/relationship' dynamic. The plot twist in the first was better, too, although it is well done in KOTOR II. However, one must have knowledge from the previous game in order to fully enjoy the experience.
KOTOR II has a half-assed ending--to be completely honest, though the original's wasn't that great, either--and, from what I can tell, LS/DS endings are nearly (if not exactly) identical. Also, in order to fully "get" the plot, one must play as a LSer, as a DSer misses out on the most integral portion of the plot.
Some of the quests in KOTOR II are really good, and one can do all sorts of little things to advance them (Dantooine is, in my opinion, THE best part of the game).
Taking everything into consideration, KOTOR II has one fatal flaw that turned me off from the game:
The time spent on the ship after one awakens from one's coma/trance is WAY too long, and there is too much backtracking.
In the end, save the game for a rental and have fun with it.