I don't think he was a bust, but I also think he never realized his full potential. Pena could have been-- had the talent and bat speed-- an All Star, but needed more time to simply play. In retrospect, Pena would have been better served to have played full time from the time he came up. Cincinnati should have dealt one of their OF or Casey to make that happen. Dealing Casey for pitching just after 1999, for example, could have been very interesting.
Having said that, this is 20/20 hindsight GMing. No way anyone could have seen this coming.