Recs.
Updated
SpecsUpdate
Pros
Pro Easier and more modern controls make playing more enjoyable
Drag and drop actions have been added for many tools, you can sort through lists, the map will scroll when the mouse is near the edge of the map, and you can estimate costs by holding the shift key. All of these don't add new things to the game, but they make managing items much easier.
Pro Familiar playstyle to Transport Tycoon Deluxe, plus enhancements
OpenTTD is based off Transport Tycoon Deluxe, however it has been tweaked and added to. The game remains familiar enough to its roots, but with many more options. Some of the tweaks are larger maps, transportation lines (autorail, canals, mammoth trains), the ability to bribe the town authority and more.
Cons
Con Can be boring when playing alone
Due to the length of the win conditions and how long it takes to achieve things in the game, it can get a bit boring after much time spent. This is also true of the stock gameplay element, while mods can definitely help, what comes with the game out of the box can get old after a while.
Con Inconvenient AI installation
In order to get an AI up and running, you have to download a mod to then install it to the game. This may be too much trouble for some people who are not familiar with installing mods, or just more time than they would like to spend on seeing an AI present in the game.
Con Online does not work cross versions
Old versions can't play online or LAN with newer versions of the game. So for those that prefer using an older version, or just have not updated, they will have to make sure those they want to play with online have the exact same version (or one that is compatible).