I seem to be one of the resident Skylander people on the forums (pretty sure 90% of the figures have been uploaded by me). Here goes
is the game worth playing? Yes. Its a decent game even if the targeted audience is around 10 years old. Only complaint Ive had is the shortness of the game and the replay of each level after you get all the stars is low.
are the figures compatible with the different versions? The figures are compatible forward 100% but not backwards at all.
Do I need to buy more figures to really get the best experience? For each game, you need a certain minimum to enjoy all the game has to offer. Spyro's - one of each of the 8 elements, Giants - Previous plus at least 1 giant, Swap force - Previous plus one of each of the 8 additional bonus swappable characters. The bonuses include sneaking, climbing, speed, etc.
If you have any other questions, let me know.