It is possible to get excellent results with the Johnny's six-row seeder, but it's fiddly and you have to be methodical about it, so it's not for everyone. It's definitely not as simple as just selecting the hole size and eyeballing the thumbscrew settings for the brushes. I recommend that you loosen the thumbscrews all the way until they stop against the acorn nuts, and then mark them so you can tell how far you have turned them and can repeat the settings once you find what works.
Figuring out what settings to use for a new crop is the most annoying and time-consuming part. I put it all the way shallow (so the furrowers aren't quite touching the ground) and do dry-runs on a big doormat to get the settings roughly correct. That takes half to three-quarters of an hour, as the hoppers are not exactly the same and you have to figure out individual settings for each brush. Then I figure it takes two or three trial plantings to see how it behaves in the soil and adjust accordingly. I write down my settings in eighths of a turn (45 degrees). That's big enough to make a noticeable difference when planting baby salad grens, and about as small as you can set accurately.
To do dry runs you want something a little soft so the seeds don't bounce out of their rows too badly, and smooth enough that the seed won't stick to it, so you can pick it up and pour the seed back into the container. I use a big doormat, the interior kind with no ridges or other texture. But you can also turn it over and use the rubber back...the seeds bounce more, but it's not too bad.
I find that you generally use a size bigger hole than you think (and than Johnny's recommends). We don't use the smallest hole for anything. I use the second hole for carrots and lettuce, and the third hole for spinach. We are planting the baby salad greens more densely than 1", so it is no longer a precision seeder; the brushes are raised so that a seed almost fits under, and then the friction of the bar is enough to push seeds out. This makes the settings very sensitive to seed size, and I have adjustments to the base settings for each variety (see my essay from last April). Spinach and carrots, where we are seeding at 1" spacing, are less sensitive to seed size.
Once you get the settings worked out, it's pretty quick and you can get excellent results. Here is some Saladbowl lettuce just coming up (along with a 12" ruler):
This Spock lettuce had some patches where it didn't come up well; I'm not sure if our salt levels were high, or there was some disease issues, or possibly both:
Here is some arugula which we just cut, and the next planting which we will cut in another week or so:
Here's a view down the greenhouse. I was trying to get mainly a shot of those two beds; the left-hand one is lettuce which will be ready to harvest in another maybe 10 days, ditto the mustard greens in the near half of the right-hand bed. In the far half of the right hand bed (across from the green lettuce) is the arugula which we just cut, and past that some mustard greens which are ready to cut. On the far left is an old planting which has been cut maybe three times and will be pulled out soon, and the edge bed we just re-planted. The beds over to the right are younger lettuce.
Here is how spinach comes up. We planted this in...late November, I think? This was a trial; we weren't really expecting it to survive. But so far it has pulled through just fine, although we have had outside temperatures of -17 Fahrenheit (inside the greenhouse was 5, and I didn't have a thermometer under the low covers). It looks pretty sad right now because it's early morning and it's still frozen, but it will be fine once things warm up. So we're expecting a bumper crop in late March or early April.
I don't have pictures of carrots, I'm afraid, but they work splendidly also. As with the spinach, we use the 1" spacing, and only put seed in every other hopper, giving rows about 4" apart. It gives beautiful plantings of carrots which often don't need any thinning at all.