[eluser]skunkbad[/eluser]
CodeIgniter does not make working with HTTPS easy for you. I had to extend many of the helper functions so that URLs, anchors, and form open tags would work in the appropriate environment. This is something that Laravel got right, but the CodeIgniter team has yet to implement. If you are interested, you can check out my helpers in the Community Auth repo.
Using a security certificate is essential if you are dealing with credit cards or other sensitive data. Sensitive data may include bank account numbers, birth dates, social security numbers, or anything the user wouldn't want to hand an identity thief. Because people basically expect you to keep their data secure, one of the biggest pros in regards to using HTTPS is that you can help keep yourself from getting sued. While I'm sure there are hackers that can do just about anything, using HTTPS means protecting all requests, responses and cookie data (provided you set the secure flag on the cookie).
