ive only ever fooled around with one, but i would say the best way to get into medium format would be to find a used Twin lens. the waist level viewfinder will REALLY teach you alot about composition and shooting. it definately takes some getting used to having a square negative as opposed to a rectangle. i have caught myself a few times almost cutting things out, or including things that i didnt want in a photo.
my only difficulty is getting used to focusing with the waistlevel. my yashica has a pop up magnifier that really helps though. good luck! i really really love medium format! at first i thought it was a PITA to roll onto a reel, now i am the other way, i have a hard time with 35mm and 120 comes second nature. good luck!