I know you have no control over aperture and that results in a lack of control. I have found that even when shooting in darker conditions the zones hold up. Things really are in focus within the zones, even at the aperture that provides the smallest depth of field. It's a lomography camera and my best advice is the advice they give in their golden rules. Lomo tells you to not think, just shoot. Go with that. The lca-120 is a pretty nice little camera. Medium format, autoexposure, very small and lightweight, protected lens so easy to throw In a bag. The metering system responds to fluctuations in lighting while the exposure is happening, not just when you first press the shutter. It's really not half bad and I enjoy mine. I can't say I've had many focus issues. If you find you like it, their lc-a+ is very similar but I find that camera shines better in well lit conditions, which produce more of the "lomo style". Heavy vignetting and very very saturated colours even without cross processing. Strangely my lca-120 only produces the vignetting. The colours for my 120 are very subdued I find. Maybe yours will be different though.
With that camera though remember to press the shutter with some authority. It does weird things with half presses.