Went to see Briget Jones: Edge of Reason at a 'Big Scream' before Christmas and really enjoyed it. It was really well attended - lots of parents with little ones in carseats. Dd slept through most of it (but she was only about 8 weeks old, and is deaf to boot, so not surprising). There were a few older babies crawling around down at the front and the odd bout of crying, but on the whole, it wasn't distracting. However, I wouldn't recommend a session at a film requiring you to concentrate hard or read subtitles - keep it lighthearted.
I did this for the first time the other day with DD (11 months) and it was great. She crawled around and played with toys and I managed to watch all of the film (Bridget Jones).
There was always at least a couple of babies crying but I reckon there were around 200 there. You get used to the noise as it's like watching Neighbours at home while getting dinner ready and your own children are being noisy; you just don't hear them!
At one point a whole row of babies set each off which was actually funny.
I thought it would be great thing to do with a really little one. Friend of friend I met there said she often went to sleep!