I’ve been wondering what the downsides are with StartCom’s free SSL certificates.

At the moment those seem to be:

• You can only renew them for 1 year – could be tedious if you
  have lots of them.
• Windows XP users need to have installed at least Service Pack 2
  to have the CA. Apparently non-updated Windows XP works now! Just in time for its EOL.
• Blackberry and other RIM devices have no support. @startssl
  says: “Correct RIM has no support so far (we understand that they
  are working on it though).”
• Reports of no support in an iPod Touch running iOS 3.x.

Useful SSL checkers:

• https://www.ssllabs.com/ssldb/analyze.html
• http://www.sslshopper.com/ssl-checker.html