I adore, literally, 6x9 format. I've always been crazy for it since my early youth. It's like having large format always with you, but in a pocketable size and with the convenience of 120 rollfilm.
I also own a Zeiss Ikon Super Ikonta 531/2. That's my standard camera for alpinism, I always carry it with me. I actually owned three Super Ikontas over the years, progressing in lens quality and camera conditions. These cameras are highly collectable and they're getting very old; finding a unit free from defects and at a reasonable price is not an easy task today. Unfortunately there are many collectors that pay big bucks for nearly perfect units, and then keep them still in a shrine. What a waste of excellent cameras.
In the past, I owned a Linhof Super Technika 6x9. Quite obviously this camera is expensive and also very heavy, but aside from that I can't say but great great things about it. That camera is "the ultimate keeper", I believe nothing better than that was made in 6x9; however with heavy heart I resold it when I stepped up to 4x5'' - the reason is that I can use a 6x9 rollfilm adapter with the 4x5'' camera, so the Linhof 6x9 basically had no more use for me. I hate to keep cameras still in a shrine (see above), so I passed it to someone else who hopingly will make great use of it.
My biggest regret in recent years is that the Voigtlander Bessa III was made 6x6 and 6x7 only. The twin format was a *brilliant* idea, but what a pity it was "only" 6x7, what a wasted chance. Why, oh why they didn't made it 6x9...