Well I've had Lowepro, Crumpler, CSS, and others and really they are all a bit over engineered for my simpleton needs. They are stiff, hard to slip stuff in and out, don't take as much you might expect on the whole, have 'steal me branding' all over them and sit in the L class of rip off for the average amateur like most of us lot.
Sure if you want the 'status' ( which is in your own head anyway: no one else know a Crumpler from a shopping bag 
) then spend the money; if you just want a nice soft bag that holds all the walkaround stuff most of the 'sensible' amongst us will need within our urban wonderland then I have the solution. I got it in Gap last week and it cost me around £12.95 in the sale 
So, specific advantages: it is nice and soft so it doesn't make a rubbing noise as you walk and doesn't rub a friction hole in your pants or set your pants on fire and thus scar you for life. It is also tough. It is like a bl**dy tardis and holds way more than you'd expect and also seems very light even when full!! It hangs off your shoulder with no effort rather than constantly slipping off, it has a mini pocket for keys or CF cards and stuff so easy access, the lid flap just throws over to close (tho' it can be secured too if needed) and flips back to open, there is a front pocket for hood/s, umbrella, PDA, mobile, MP3 and more. Easy access there too, which is vital for street shooting. The strap is strong and soft so very comfortable to walk with.
It has two zipped pockets one each side. In one I have my Pentax 50mm 1.4 (the Light Maker) and my Zeiss 35mm 2.4 (the Bokah Bonker) and could easily fit a third (these MF's are tiny remember!) and in the other pocket I have my sweet Pentax 200mm 4.0 (the Mini Marvel).
Inside, I have my ultra reliable Sigma 15-30mm 3.5-4.5 (the L Breaker) and then my 30D with 24-105mm 4.0 IS L attached (the Plasticy Saturator). Which ever I am not using goes in the L's crummy but effective soft pouch.
Security ... well you can clasp the lid closed with the two ... erm ... clasps and because the zips for either side pocket are on soft material you have to hold the bag to open easily. Hard to open sneak thief style (I learnt my urban pick pocketing thievery compensation techniques in India, where some folks are masters of the craft, so I have an idea what to watch for in a bag and generally!!) Sure they could try knifing it open but the bag is so soft you would be more likely to feel the movement.
The only thing against it is the lack of internal floor padding, but you can easy get a piece of cheap foam strip and drop it in. I use a thickish double sheet of bubble wrap to take any of the normal day to day pick up and put down impact. Sure if you drop it then things will jar. But if you are half witted in that kinda way then add some foam padding. I very rarely drop things or poke lenses with branches or bang my camera into brick walls (never actually!), tho I know some here do it almost daily it seems
So, the proof of the pudding is in the eating ... and while I haven't eaten this bag I have taken it on a number of walkarounds where normally I would leave the camera and stuff at home. My big Crumpler is a truly great 'vanity photographer' bag, but functional for long trips and flights more than day to day walkaround. I have used it three times in as many months. I have used the Gap bag 4 times this week
Franny doesn't come with the bag BTW. She just liked hanging around. I think she fancies herself as a model
