Alibaba itself is a B2B directory, within that directory you have individual suppliers. As with any B2B directory, you'll find good and bad suppliers in those directories. So you really have to do your research on the suppliers within the directory, not on Alibaba itself.
So as I said, you'll find a mix of good and bad, but as with any supplier, you have to do your research and even when you're happy to proceed, make sure you choose a smart payment option. PayPal of Credit Card to give yourself some protection. Never go with a money transfer of bank wire, that's when you are putting your money at serious risk.