I wandered into one of these 'festivals' once when I was in SSP. As Shri points out, it tends to be loads of indistinguishable stores sellging largely identical products to large crowds.
If I recall correctly, the products tend to be standard consumer items such as printers, external hard drives, screens and laptops. I don't recally seeing components such as RAM or graphics cards, but maybe they are pushed too.
If you do go, you should know exactly what you want (brand, model, spec) and for how much. There is little opportunity to browse comfortably, and little opportunity to ask sales people questions (not that you should be asking salespeople tech questions in HK anyway). I rather suspect that deals are hard to come by, but the only basis I have for this claim is that the USB thumb drives that were being pushed at the last fair I went to were going for normal retail prices anyway. Margins in HK are so low that I don't think there is much scope for additional discounting.