No, I am not going to tell you how to operate your latest gadget. You have that nice little booklet that came with it, after all - even if we both know you haven't read it (you should, it can sometimes be an eye-opening exercise). Also, how on Earth would I know which brand you have (mine's a Nokia, so it does so much more, too)? No, what I propose to do is tell you what I found works well (and not so well) for me. This was inspired by an interesting entry about traffic updates and their real utility. I know the article was slightly tongue-in-cheek, but it set me thinking (it doesn't take much), and here's what the result was.
I'll first tell you why I never let SatNav automatically re-route depending on traffic updates it receives. Yes, in the final analysis, it is for the same end reason the author of the inspirational piece above got annoyed with this "feature". I propose to tell you why it can be annoying, and why you'll, more often than not, be better off without it. I outlined the argument very briefly in the comments section for The Economist piece, but here's some more details.
By their very nature traffic updates can never be "live". Data in them takes some time to collect, even if this is a running exercise. It also takes at least some time to compile into usable report, send it out, and then for it to be processed by your SatNav. While most of the time traffic conditions do not change suddenly, it is precisely when they do that you'd want your traffic updates enabled SatNav to be able to help, and that's precisely when it will most likely fail you.
Imagine a rush hour (yes, I know it's not very nice even thinking about it, but just for a moment). It is true that any current congestion is unlikely to clear up quickly. However, a new congested spot gets created in an instant, and as much as your SatNav was good at directing you away and around the old ones, you'll still find yourself driving straight into a new one, with your SatNav blissfully unaware of it for some time. And then, when the "live" traffic update finally arrives, it will re-route you again, maybe still unaware of some spots that cleared by now. In all likelihood it will make you zigzag in a seemingly random fashion, with your bonnet rarely pointing in the general direction you wanted to go in the first place. In the end you may find you spent longer trying to avoid delays than if you stuck to your guns and plodded on regardless.
Do understand (I do) that the above is not saying anyone in the SatNav R&D department has got anything wrong. They are simply doing the best with what they have. It's just that this is sometimes not good enough. And when it fails it annoys you so much that you forget all the other benefits of the technology. I also do not propose I have a solution to make this particular SatNav function better. Even if I think there's precious little that can be done here, this is so far from my area(s) of "expertise" that it would be almost rude to wade there. Instead what I propose to do is to point out how to get the best out of an imperfect technology (mostly by relying on another, ancient one).
So, first, do turn off re-routing based on traffic updates. There's no reason to turn off traffic updates themselves, especially if your SatNav has a way of notifying you of them that is unobtrusive yet informative. For example, I'd love them to make congested stretches of roads "glow" softly red or something like that. A voice warning if congestion is some miles ahead on the road you're on is also of great help. Even offering to re-route around congestion at a press of a button would be OK. It's just that I think that you should be in control of the route at all times.
And this brings us to what a lot of people are forgetting these days: paper maps, and general awareness of where one should be heading (and the available options). Yes, most of the time you can just jump into your car, punch in an address into your SatNav, and find yourself at your chosen point B without any hassle. But that's exactly one of my points. When everything is running smoothly (or close enough not to matter) even traffic updates are fine. It's just that when things start getting seriously wrong you'll be well advised to a) have a paper map with you, and b) be able to use it. Of course, the best policy would be to survey the map before you leave, and memorise a few alternative routes and rough points where they can be taken. That way you don't need to break your journey to study the map in the cramped car. Or get out of it to buy one from your friendly (and extortionately expensive) local garage.
Lastly, just like computers are really much better at being an eraser than a pencil, the SatNav main strength for me is it's ability to route you out of a wrong turn, even if it takes you into a totally unknown part of wherever you are. So, if, in trying to avoid a traffic jam in front of you, you make dog's breakfast of the alternative route, at least you can rest assured that you will eventually be extricated from it without the need to stop, study the map, or - shock! horror! - as some innocent bystander.
In short, a SatNav is good from taking you from your intended A to B. There is no doubt about that (provided you use it wisely, i.e. not relying on re-routing due to traffic updates). However, where SatNavs excel is getting you to your intended B, once you have hopelessly lost yourself, with no idea where your current A is.
My conclusion and advice: do use SatNavs - they're great gadgets. You just need to use them wisely, aware of their limitations even more than their other "features". Also, do not throw away that paper map, but make sure it's in your car. And use it, too. Your neck-top computer is still way more powerful than anything else humankind came up with. Yet.