I've used them in the past with no problem (US pre9/11, Europe Post 9/11) but I'm about to head off this weekend for 5 weeks in Europe and I don’t think I'll bother with the bag any more, I think if films were getting toasted you would hear about it big time. I'll be carrying 400 & 800 colour print, some of it may get scanned about eight times.
Just out of interest ...I've only had my bag stopped three times in maybe 20+ airport transits... once in the US pre9/11, they wanted to see what it was, they read the label on the bag and gave it back to me, I asked if the wanted me to open it for them and they said “no thanks”

.... once in France post 9/11 getting on the eurostar train to the UK I was asked to show the bag and its contents, (Inside I had the film in a clear ziplock bag with white fuji see thru canisters) they held up the inner bag for a look and said fine and off I go with no further scans

.... Coming back home to Australia (post 9/11 and who are normally the most uptight airport/customs people I've come across) I see my bag draws a few people at the monitor and I get asked if they can check my bag, I'm like "sure" and then they go and check the wrong backpack

after a few minutes of fruitless searching and the guys face going a little red he said "that fine you can carry on", I would have helped if he asked but after just under 20hrs in cattle class I wasn’t feeling very generous
In most of the world I could go either way with using a lead bag but I wouldn't in the US at all .. It's not worth the grief IMO
EDIT: Worth mentioning, film must go in your carry-on bag !! Any that go in you checked suitcases will be tosted with or without a lead bag !!