Apart from its quirks Excel is fine, if you know its limitations. I think the real warning sign would be an analyst/programmer working with Excel and expecting high precision results.
Everything is fine apart from its quirks, if you know its limitations. The problem is when a tool's quirks and limitations are neither exhaustively documented nor can they be inferred from having reasonable knowledge about the base principles of the tool, but are rather learnt by experience (to be read as "through bad experiences").
