I can't speak for anyone but myself, but i had the cheapest of cheap opks from Ebay, and they were very sensitive ones. I was getting mainly dark(ish) lines right through my cycle but got an obvious positive on CD13 and 14 where the test line was so much darker than control line, so the cheapies worked for me. I backed this up with Asda's own opks (5 in a pack) and they were great too!
i think if you used the cheapies definitely back it up with something else, either another brand or a digi. Alot of the girls swear by digis.
With regard to the hpts, i don't think anyone would recommend the internet cheapies, but they DO work well for some. I buy them to fulfill my addiction of poas! Again for testing I would recommend the FRER or superdrugs own brand xxxxxx
p.s. GOOD LUCK!!!!!!!!!!!