At the last ones, there were two showings - one at 5pm and one at 7pm. The 5pm ones were for industry types and bods who couldn't make it to the 7pm. I can't speak for everyone's experiences, but the 5pm showing for mine was like a frikkin' morgue - everyone was sober and nobody wanted to be the first to laugh.
In comparison, my 7pm showing was like Mardi Gras, a U2 concert and the end of Return of the Jedi all rolled into one (but without Bono and most of the Ewoks) in comparison. People had had a few drinks and the atmosphere was much more convivial.
The audience was pretty split between friends of the writers / cast, industry types and just generally interested comedy fans. But as Dan / SwertyD has said, if it's funny, then people will laugh despite themselves.
However, at some of the performances, you could hear some forced laughter from partisan camps within the audience. So if your's does get selected, be sure to get tickets and bring some mates, as any support will only go in your favour.
As a super plus mega bonus, I got to meet several of the posters here on the BCG and what a lovely bunch of coconuts people they turned out to be.