Getting a third party cert really wouldn't provide much extra evidence that you have in fact reached the correct web site. And for that small benefit, I don't find it worth participating in Verisign's scam to extort money out of businesses while promising that its certs do more than they actually do in practice. If Verisign signed certs were really about providing security, rather than cash, to Verisign, the way browsers handled certificate manangement would be different.
The SSL connection has some extra overhead and will be slightly slower than if you weren't using it.
It keeps the national intelligence agencies (FBI, CIA, NSA, etc.) and your ISP (especially COMCAST) from seeing what on my site you are looking at. (They can still tell you are looking at something here, and if an intelligence agency is particularly interested in either you or me, then they have ways of getting more infromation by compromising the end points.) These groups think it is OK for them to record anyone's traffic without being held accountable. If you disaggree with that stance, you should to do something about it.
For a small group of people (pretty much just AREA helpers), this will be used to authenticate their access to the site.