Buses run from one park to another. For Epcot to Magic Kingdom, use the monorail. For all Magic Kingdom trips you still have to take the express monorail or ferry to/from the Transportation and Ticket Center (MK parking lot area) where the park to park buses board.
You can take a boat from MGM Studios to the rear of Epcot, but if the boat is not at the dock or arriving for you, you can walk the distance in less time.
If we are going from Epcot to the MK or visa versa, we take the monorail. If we are going from Epcot to MGM or visa versa we take the Friendship boats. If we are going between any of the other parks, we take the busses. The only time you will have to make an intermediary stop at a resort is if you are traveling to or from Downtown Disney.
The only place I've ever had a hard time getting a bus was at the TTC. Kind of ironic, since back in the day that was really the central hub of WDW. These days, in the middle of the day it's a ghost town. One day I waited 45 minutes with another family that said they had been waiting 15 before I got there - to get to AK. Other busses were infrequent as well.
Of course to Epcot the best bet is the monorail, but to park hop from MK to MGM or AK from then on I found it best to take the bus to the resort and then from the resort to the park. I usually used the opportunity to fill up my mug as well.
Other than that, I've found the bus service between parks and everywhere else great. And probably I just hit a bad day at the TTC - I just use it as an excuse because I just love getting free drink refills.