I dislike camera bags in general because I normally prefer to carry my camera in my hand or over my shoulder. Rarely do I carry spare lenses and filters, a spare roll of film fits in the pocket easily. However, every now and then I'll go somewhere special and I'll want to carry more gear. Camera bags tend to be over kill for this as they're often designed for a war zone rather than the 8:15 to Euston. What's most important for me is to protect the equipment from itself - things hitting each other.
With this in mind I discovered the Tenba photo insert which is ideal if you need to convert a normal messenger bag into a temporary photo bag. This is the bag I use for work, I chose it because it fits an A4 folder and doesn't have a velcro flap:
And here's the Tenba:
It fits perfectly into my bag:
The only issue is that there's no padding on top, but for my sort of photography that's not a real concern and anyway when the zip is closed it pulls the top of the insert inward to a degree. If you shoot with a small 35mm SLR like me, the Tenba is a little too deep:
But nevertheless this is a great option for someone who usually doesn't use a bag, it will easily fit an SLR and 3-4 lenses, maybe a little less if you have huge telephotos or those massive DSLRS which are larger than your face. In action, the insert fits well and stays put, the bag remains fairly narrow so it doesn't feel like you've got a suitcase strapped to your hip. In all a great buy.