Is D3 worth the $55 upgrade from D2?

[LDiCesare]

Dom3, from a SP point of view (because MP is really a no-brainer), adds the following:
1) Much better interface.
I really mean it. It's still ugly as hell, but I went back to Dom 2 not so long ago to test astral weapon, and it feels clunky.
The things to consider include:
1.1) Automated tax management.
1.2) Units displayed on the map instead of big squares.
1.2) New shortcuts like shift-Z to send your bloodslaves to the lab.
2) AI builds forts.
This means the ai is no longer a pitiful pushover. I mean, it is, but if you put a lot of incredible ai's in your game, Some of them are bound to cause you problems.
3) New content.
There are many new nations, many of them added in later patches (Lanka, Fomoria, Hinnom, Gath, Bogarus and more)

The latter is enough to totally change SP on any but the smallest maps, and well worth the price.