Palantir Technologies (PLTR -4.00%) has been one of the biggest winners of the artificial intelligence (AI) boom. It led the S&P 500 (^GSPC -1.61%) last year with a 340% gain, and it is currently the second-best performing stock in the index this year, with a 66% gain.Palantir is particularly popular with retail investors, many of whom admire CEO Alex Karp for his forthright communication and unapologetic viewpoints. But after tenfold returns in the last two years, valuation has become a serious concern for ...