Looking for opinions here ... is it better to purchase an SSL certificate, or to use OpenSSL to create one?

What's the difference?

well, if you create it yourself using openssl, you will get the certificate warning when trying to access secured pages by web browser.... instead, if you buy thawte,geotrust, verisign they are recognized by major web browsers and will not generate a certificate warning message

I personally use the geotrust certificates as I collect infomation from my gateway site, pass to merchant then variables are passed back to the hosting site - so 2 warnings for a single transaction would be a pain in the arse for a user

BTW; another misconception regarding certificates is that if a user signs up to a site without a registered certficate, they often assume their credit card information is not as safe as a company with a registered cert.