We've been on 5
DCL trips and have purchased our now ds10 and ds6 tuxedo's and suits 3 times on ebay for our two 7 day cruises and 11 day Med cruise. We didn't have them wear tuxedos on our 3 and 4 day cruises. We were very happy with the tux's we received. They only wore them once before they grew out of them, but for the price it was worth it. DS10's sport coat cost more than his tux on our 1st 7 day. After that we bought suits from the same person on ebay. Like other posters have said, be careful in measuring. My ds10's tux was a size 12 and his suit was a 10 but both fit him perfect. My parents had cummerbunds and bow ties made out of Mickey material on our 1st 7 day that they have worn each time.
Here's a picture of the boys in their suits/tuxedos from our Mediterranean cruise in July 2007: