With the right tyres they're a very fun challenge. If I'm fortunate enough to have something like a Hans Dampf on, they're good fun!
Generally you should be thinking about what's after the roots rather than the roots themselves – I generally try to unweight the bike and focus on the half decent bit of trail ahead of them where I can plant/chuck the bike into.
Another one I find is that if you can ride them higher up and closer to the base of the trunk where they're wider then that helps. Generally it's not the tread that slips but if you get enough root in contact with the tyre carcass. So the narrower the root, the more likely it is to get between the nobbles and slide on the carcass. Baring that in mind, I find tyres with a more MX style tread pattern (Blue Groove, Hans Dampf) tend to go better on roots as there are less horizontal lines/gaps all the way along the tread.
Obv 90 degrees if possible but not always possible.