On impulse I bought a Holga 120, cheap enough from Freestyle, overpriced seriously in Oz. I had achieved quite good results with a RB67 and a Yashicamat 124G via the slide across and make the second exposure technique. That has the disadvantage of course that any moving object becomes a hazy ghost in the viewed stereo print.
The Holga applies tension to the film with a cheap foam plastic insert on either side. These pulled off on the first advance wind, stuck in the gate and wrecked most exposures.
The salvaged ones were what I guess Holga is known for: flat contrast and overall fuzzy focus. They did save me the task of cutting and aligning the contact prints.
I feel that the stereo effect is best achieved when the prints are sharp and have good contrast, the Holga isn't great for that. Manfrotto makes a good slide bar with a scale that works accurately and well with the slide across technique.