I have a a Flipside 300 and have used it for a year to carry a tripod, filters, Tokina 11-16, Canon 70-200 for long walks up and down hills and has served me well. It even does a decent job of resisting water, though I do own a rain cover for it that I use just incase. But I did get caught ina summer downpour and was wet on outside, but bone dry on inside.
I am now moving to a new setup with Canon 5d MK II, Canon 17-40 and Canon 24-70. I have bought a Vertex 200 and it's a massive bag. Sliplock features are a bit rubbish on it, but the storage is immense. Even fit some clothing or stuff into the laptop pocket.
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My only complaint about the Flipside 300 is even though you can day walk with it, your shoulders may ache depending on the weight load. Make sure you use the distribution straps to evenly spreadout weight if you get a Flipside 300. For the price they are very good bags even if it's to carry a bit of gear. Small, compact and will do a decent job.
The vertex system looks pretty decent in terms of spreading out weight, but bags are funny. Depends on the person and gear. I got mine online, but if you can go to a store and have a look or even try one on then that will be beneficial to you.