It's difficult to know quite how to rate this game. In many ways, it is an improvement over its predecessor, the excellent SK1, with new buildings, new units, new civilisations, and a whole new way to play (when using the dreaded Frythans). For someone who's experienced everything the first game had to offer, this maintains the same foundations while building all kinds of cool stuff on top of them.
Unfortunately, in terms of the way the game plays out, this is not an unequivocally good thing. All the extra stuff begins to feel a bit cluttered, so that where in the first game there were just a few clear strategic paths that one could try to follow to victory, this one can be a bit overwhelming in the number of decisions the player is called on to make - and the speed they're expected to make them at. Also, I have to admit to liking the graphics in the first game better. While they were a little basic compared to today's offerings - and indeed, this one is basic compared with a game like Company of Heroes, too - in SK1 you could always quickly and easily see what was going on. The graphics in this one feel a bit more "busy", which can make it harder to work out exactly where your men are or how many of them you have or whether you're winning or losing a battle.
Still, there have also been some interface improvements in this version, in addition to the aforementioned additions of "cool stuff", so for anyone who enjoyed the first game, I would still recommend trying this one out. If you're on the fence, or haven't played either game before, I'd say, start with the first one, which is a really outstanding and innovative game in its own right, and if you like that one enough, you'll probably end up wanting to try out this one too.