PPT games in my experience so far have been great! This coming from someone who has always been suspicious and challenged in the technological realm; I now have to say I am a firm believer in its usefulness as a teaching tool.
I don't use ppt games all the time, but when I do the whole class participates (which is very unusual in the big middle school where I teach students of varying - some of extremely low - English ability) and they get excited about learning. I think competition is a great motivator, but so are the graphics - the students just can't wait to see the bomb or typhoon pop up after a team's answer. They learn different English words ('pass;' 'your time is up!'; 'typhoon' - which has led to a general discussion on weather and weather terminology before, etc, etc)
I think the main issues are that you need to ask thought-provoking, conversational questions, and you need to still be a teacher - facilitate the process of learning through games by adding extra information/asking other questions where necessary, etc. -basically, don't rely ONLY on the ppt. It is only meant to be a tool - not the teacher.
Also, some of the posters mentioned that it doesn't get their students talking...try to initiate a rule whereby if anyone is caught speaking Korean they get a minus point. Then play the games in teams, so necessitating discussion amongst team members. The students will police each other, I assure you! - mine do. They will do anything to win, and they are constantly telling their classmates 'English-ee, John' (I don't know why, but they always call each other John when they are speaking English...some kind of inside joke!)
Thank you everyone for the fantastic ppt games - my students love them!