Viking Invasion is a very good addition to the MTW. Smaller map and vikings make it feel quite different from medieval.

IMO, the major difference between MTW before and after the Viking is that ( generally) you cannot pillage a province without taking its castle. Makes a big difference because the AI is able to build empires now. Another nice feature is the pre-battle screen which makes planning for the battle much easier.

In general, I think that the AI is slightly better after Viking (both in the new Campaign and in the Main Campaign).