Worth getting if I already have the original?

[Division By Zero]

I already have the original Age of Wonders (on GOG). How much more does this game add upon the original?

I'm seeing a lot of difficulty complaints and it looks like damage is... random with no minimum???

(Probably should have asked this later as the sale will be over once someone replies, however I can always wait for the next sale).

[5M0k1N]

it has new campaigns and some more stuff. if you already played through the original and want more then its definately worth it

[PXR5]

Yes it is worth it. AoW:SM is the best in the series. Especially with the community patch. A lot of fan made maps that surpas the original maps and are still better than the latest AoW3

[Chimaera]

It has flavor text for all units that I can think of, in addition to new skills and spells and structures.
If you like mods by any chance then there are plenty of them out there and unlike the original it has a random map generator (which works very well). If you did not get it yet, I recommend it.