Lynsey/Julia,
Not yet been to Prague or Vienna but they are on my "to do" list.
Although i tend to do city breaks independant not with JY I can tell you i really enjoyed Budapest. My favorite part of the city is the castle district on the Buda side of the river.
I enjoyed the food in Budapest but one piece of advice is don't use the touristy restaurants on the touristy pedestrian Vaci Utca not there is anything wrong with the food but you will get far better it the small places down some of the small side streets.
Other bits of advice is I found i got a much better exchange rate for Florint in the many exchange booths around the city than in the UK.
A must visit is the Terror museum, not nice history and tells you about what happend during both the German & Russian occupation, its in the building that was used by both regimes secret police.
Mick