I’ve only just remembered that I’ve never written anything about my attempt at using Tchibo straight knitting loom. It was a disaster. I actually went there and returned it (got all my money back, good Tchibo policy here).
It was soft and flexible. VERY soft. When I put the first round of yarn on it, the sides bent into the middle. Whatever I did, I couldn’t make any kind of consistent knit, it all came out as a bunch of very very loose loops.
Look from the top (see how it bends into the middle?):
Look from the side (only a few rows, but even when I made some more in hope of this getting a bit tighter, no luck):
But as you can see in the second photo, it’s all fault of very poor construction – the frame itself is hollow and walls are very thin. The plastic bent under slightest pressure, including loosely threaded yarn.
In summary: when buying a knitting loom, make sure the frame is solid (wood is good!). PRYM round looms I’ve tried out are very sturdy. I’d like to find a straight one of the same class. Possibly with pins much closer together.