The U-Bahn is the underground system in cities. It's a bit crap that the ticket doesn't include the U-Bahn since in most cities (including Munich) you need to use the
U-Bahn to get to the actual stadium. The reason is that the U-Bahns are not operated by Deutsche Bahn. You will be able to use the
S-Bahn (longer distance urban trains) as these are operated by Deutsche Bahn.
With regards to prices and times, well as I mentioned, to Berlin from Munich it's about 9 hours instead of 6. The Schönes Wochenende Ticket is great, but it's more hassle buying the Länder Tickets if you travel during the week. You need a separate Ticket for each Land (state), so the price adds up if you're travelling a long way. Munich to Cologne, for example, would require three tickets, costing a total of about €60.
If you know where and when you'll be travelling, it may be cheapest just to book the tickets in advance (buy at least a couple of months in advance for the cheapest seats). But if you'll be doing a lot of travelling around and/or don't want to commit to a particular train 3 months in advance, then the Weltmeister Pass sounds like an OK deal.
The other options to consider are the Europass and Interrail tickets already mentioned earlier in this thread. If you're under 31 (or whatever the age limit is), this may be a better deal.