E-commerce packages vs bespoke build?

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I am looking at taking on a e-commerce site, and what was wondering what everyone thoughts on e-commerce package vs bespoke solutions are, is it beneficial to just use a 'plug+play' system like osCommerce, or maybe using something a bit more robust but with a fee and use something like Magneto? Or would it be better for just to site down plan the e-commerce enviroment out and get programming? My knowledge of packages out there is a little loose to say the least so I hope you people can provide with a good grounding of what is good, bad and indifferent?I am looking for something on a PHP platform