I bought a cheapo ROE spanner set at Super Cheap about a year ago. All of my tools were metric, and I needed some imperial ones to work on the P76 engine. Well, I got what I paid for - I have bent 2 spanners, and they don't fit on the bolts really well. Good to get you out of a pinch, but I wouldn't trust them on anything too precious.
OTOH, I left my good allen key set at a wreckers many years ago (I know what you mean, Green 510). Of course I realised my mistake several weeks later when I was replacing back brake pads on my R31, and I had to strip one of the calipers. Being Sunday afternoon, the only tool shop open near me was one of these nasty bargain warehouses.
I figured if the $2.50 allen key set let me finish the job, then it would be money well spent. I still have that same allen key set, after about 11 years of abuse. Best $2.50 tools I've ever bought