Well, I haven't been using much photography equipment so far, so I haven't got much experience with different gear.
Still, I was pretty disappointed with the Voigtländer 35mm f/1.4 Nokton MC, for which I had high hopes when I first purchased it. While being very compact and well-made, my copy exhibited massive focus shift in various directions, so I rarely got my subject in focus. The bokeh appeared very harsh and jittery to me, and I didn't like it at all. Also, the lens was rather soft wide open and both colours and contrast weren't very pleasing to me.
Another huge disappointment was that japanese-made Helios viewfinder, sporting alleged 35, 85 and 135mm framelines. I bought it to be used with the 20mm f/1.7 lens on my Olympus E-P1. It was a terrible and not very recommendable experience. The finder wouldn't fit in the flash shoe properly, and the framelines were far too tight. I could've just taken pictures blind instead.