I was wondering about this matter since I developed the What's New webpart (like Jan did as well with the The What's New Web Part for SharePoint 2007) a couple of months ago but never took the time to do some metrics to see what the best way to go was. Fortunately there is Waldek Mastykarz! He posted a very nice post Content Query Web Part vs. Custom Aggregation Web Part with graphs and a very detailed plan what all the differences are and what the best solution is to aggregate information.