Was staying in an atmospheric sort of hotel along Haarlemmerstraat that didn't include breakfast in the deal, so I walked the 10 minutes or so to check this place out, based on the Trip Advisor reviews. It would prove good enough to get three subsequent visits, plus one to the other location.
Smartest thing I did was get there for the 9:30 a.m. opening on all but one of the days. The place might seat 20-25 inside if fully loaded (weather was too chilly & drizzly for outside), so that's likely part of the angst for some. And, as everywhere, if there's a wave of orders you want to be toward the front rather than the back, as it can overload the servers and slow things down.
Sometimes you hit such a great menu item that you're confident something else on the menu can't beat it, and for me it was the eggs royale (benedict except salmon rather than ham, though they also have eggs benedict and it's good). That was 8-9 euros, with the grilled mushroom add-on at 1.20. Absolutely fantastic mix of flavors, all fresh-tasting. Can't recommend the eggs royale enough.
Every day I got a different pot of tea at 2.40 euros to wash the food down, and all were good (Earl Grey, Hapsang Souchong (sorry if I butchered that), jasmine, darjeeling, roiboos...all are served loose-leaf in a pot and poured through a strainer...very old school.
I got to peek at what others were ordering and it all looked good, from the English breakfast to the Muesli with fruit. I did try the apple crumble as a dessert one day and it was done up very well.
Wish I could have tried the high tea, but was always running around somewhere else during the hours. Dropped in on the Keizersgracht location once and snagged the last table. Their apple crumble wasn't quite as good as at Singel.
With the place as popular as it is, it's important to go early or go late or just try to avoid crowds. I knew, for example, that if I wanted to get seated on a weekend I needed to get there at opening. If you've got one or two people you might be able to wait it out or luck into something, but larger groups are tougher to seat. Service speed can be a problem at times as well if they get a surge of orders and/or are shorthanded. For many Dutch, especially on weekends, the meal's an event and not to be hurried, but of course tourists will have a different mindset.
For food quality and, to some extent, old-world atmosphere and certainly price, this is one not to be missed.