Around a year ago, I had a hankering to play settlers again. I was a great fan of Settlers 2 when it was still a newish game, and I was looking forwarding to playing that kind of game again. (Designing efficient and productive towns; exploring; expanding; and ultimately conquering my enemies.)
I decided to get Settlers 3 – just because I figured it would build on the successful formula of Settlers 2, and perhaps offer me a bit of new fun in addition to nostalgia. When I started playing it, I was immediately disappointed that the town-planning side of the game had been completely gimped. Settlers 3 doesn't use roads, and instead just had a bunch of 'carrier' settlers walking wherever they feel like to carry stuff.
Settlers 3 wasn't really the game I was looking for, but I persevered with it anyway and I had some fun. It's not a bad game, but it wasn't the kind of game I thought I was buying. It plays more like a generic Warcraft-style RTS game rather than a Settlers game. Also, the gameplay seems a bit unpolished or poorly thought through in some parts. For example, 'markets' (and thus donkeys) seem to be a complete waste of time as far as I can tell. It's trivially easy to connect different territories by using pioneers - and once that is done, ordinary carriers are generally cheaper and more effective than markets/donkeys.
Another example of unpolished gameplay is the super-cheesy territory expansion of pioneers. I'll briefly try to explain what I think is bad about pioneers. If an enemy captures one of your your military building the surrounding territory is immediately converted into being their territory. Any non-military buildings in the converted territory are immediately destroyed – which can be a significant economic blow. But the thing is, military units can only attack military buildings; and pioneers are able to expand territory far far beyond any military buildings. So by using pioneers, it's possible to have a huge amount of territory with no military buildings - leaving the enemy nothing to capture. They can try to reclaim territory using their own pioneers, or by trying to build their own military buildings on your borders - but that is extremely slow and is nowhere near as damaging as having a military building captured.
The game doesn't allow the player to have zero military buildings - but that's beside the point. If the only military buildings you have are in extremely highly defended areas, then it's very difficult for the enemy to invade. And with the fog of war - they might not even be able to work out where your military buildings even are!
... So yeah, I reckon the territory expansion mechanics in Settlers 3 are bork. Also, the AI is weak.
Recently though, I decided to try again to get the Settlers experience that I was looking for in the first place. I bought Settlers 2, 10th anniversary edition; and it's excellent. There are a few user-interface nuances, but aside from that I'm very happy with it.
To anyone thinking about which Settlers game to buy, I highly recommend the 10th anniversary edition of S2. In my opinion, Settlers 3 has removed the parts of the game the which made the Settlers series interesting and unique, and replaced those parts with mediocrity. I'm under the impression that the newer Settlers build on S3 rather than returning to their roots. - So to me the choice is clear. Settlers 2, 10th A. ed.