Today, in When in doubt, walk in, Nick suggests you try ignoring online booking systems for restaurants and try just walking in.
This may not work immediately as we’re coming up to the busiest season of all for restaurants, but he is pretty sure it will work come the bleaker month of January.
Do share your experiences of trying your luck by simply turning up by adding a post to this thread.
We recently tried it in Munich, DE, and had no luck. However, a couple right before us got lucky as I was still waiting outside for my partner.
The staff was really friendly and seemed sincerly sorry.
Tried another restaurant and this time it worked although we had to wait 15 minutes or so. Still, will do more often in the future and great article. 
I can honestly say I generally don’t bother to book unless going to a restaurant that I know is likely busy. Even then, sometimes it turns out to be unnecessary.
The only times recently this has been a cause for regret have been on trips to France and Switzerland. In Dijon, I planned to re-visit Restaurant So but they were having a ‘fermeture exceptionnelle’ both nights I visited and my spur-of-the-moment second choice was full. On a trip to Graubünden, I missed out on a pizza restaurant in Davos that was surprisingly popular. These were both spur-of-the-moment decisions, though, so I don’t think I could have booked in advance. In Dijon at least I might have booked if I had known that Restaurant So would be closed, something I’d have found out if I tried to book in advance. In general, however, I’ve mostly had good luck walking in here in Südtirol, and mostly did on these recent trips to France and Switzerland.
Having said that, I often eat lunch with my colleagues and I know they sometimes book in advance for that, again because the restaurant can get a bit crowded, and because we are sometimes a larger group (3-5 people).