Martin
I found them very good last year for radishes – I actually used the soil blocks direct in a no-dig bed (you just make a narrow trench the depth of the block and line them up next to each other). The radishes all grew very uniformly in terms of size and quality. I used two seeds per one-inch soil block.
I also tried using them for beetroots and it was OK – I then transplanted them into a few patches not used for anything else and got reasonable crops.
Whether they are actually superior to plastic trays with modules in them, I don’t know. I’ve not done any formal experiments.