I wouldn't necessarily that I've engaged in debates with groups in a public setting like I think you're talking about. I have however debated with people via facebook comments, which is kinda lame I admit, but it's still spreading rationality and reason. I've had Pascal's Wager used on me a few times, ad hominem attacks and the like. I've even had one person who backed down and said that she had to do their research on the meanings of scriptural references I cited to show the dangers of the literal interpretation of the Christian bubble, errrr, Bible. Of course, the person kept claiming that I was misinterpreting the scriptures in reference to murdering homosexuals and non-believers, stating that the quoted verses referred to "spiritual" death. Then I cited Deuteronomy 13:6-11. That's the point when she backed off and said to expect a response after she did her research. I still haven't received a response, and that's been about 4 weeks ago. I really don't like using scriptural references in debates, unless it's a debate about how dangerous the literal interpretation of them can be. In the context of taking the Bible literally, both sides usually agree on what the most literal interpretation would be, otherwise you start up a debate on the interpretation and semantics of the verse(s) in question.

Most of the people I have debated have been the uninformed or misinformed believers, which are the most common types and the easiest to deal with. As far as the usual arguments go, of the one's I've been presented with the most, Pascal's Wager is one of the most common. Followed by the free-will argument as a response to the argument from evil. I have a refutation to that one that uses the alleged God's omniscience and omni-benevolence, which it helps to agree on what the definition of omniscience is, the complete knowledge of everything without exceptions. Most of my arguments lie within the realm of philosophy, which definitely limits the theistic arguments that come from common Christians. To my knowledge, there aren't many people that are familiar with Decartes' or St. Anselm's ontological argument, or even their modern interpretations.

As far as hardcore debates go, I can't say that I haven't had any. I have however had informal, friendly debates on the subject.

I was planning to do some anti-E&R next summer holidays. E&R is Evangelism and Recreation, and means that you go on a holiday and connect to people to be able to evangelise. Now, we can do the same but, instead, tell people how the world really works and how not to be converted to religions. Or publicly debate people on camp sites, to show that you know a lot more than the average christian, and that therefore, everybody who listens, shouldn't become religious.

But that's just an idea; I haven't really debated anyone for real, and I don't know if I'll be able to do this, either