Almost as good as the real thing
Still my favorite calculator of all time--since freshman year in 1985, anyway, and my 15c stills work perfectly. But this app is the next best thing, for when Im not in my office. One star off for the $15 price though. (To the people complaining about the clicking, go to the iPad/iPhone settings and there is a preference for the 15c where you can turn off the clicks, set rotation lock, etc.)
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HP 15C Calculator