I recently. over the last few months, have been using the oscommerce system to run the store for a client of mine. We recently switched over to using a straight paypal shopping cart to take payments.Here is why:
Often, we would visit the store and find pages down. This was probably do to hosting problems.
When it came to dealing with installing our payment processor, we spoke repetedly with our creditcard processing company (that was supposed to be intergated in to the oscommerce store) and to our webhosting company, with no results. The only support I could find was in the form of a forum, and modifications to these stores are really complicated, and require a good knowledge of php.
Since we could not get our CC processor to work at all, and no support to get the problem fixed, we desided to go with a simple paypal setup. No problems there, and now we are seeing sales. Hmmm?
Others using other hosting services, and other CC processors probably wouldnt have the same problems, but unless you or your webmaster are an expert in php, I would suggest using another store system.