There are some terrible downpipes on ebay, but not all of them are bad. The 4 main problems I've seen are:
1) Flexi too short as already mentioned.
2) The front hanger bar is welded too far back.
3) The internal bore diameter is too small. The turbo outlet ID is about 85mm. 3" OD is 76mm, so it can create a step in the flow. R Tech have seen some that are only 2.5" ID that are sold as 3". This causes a big enough restriction to cause valve float.
4) The pipe thickness is a bit thin, so it sounds like it's blowing, but it isn't.
I went with the APR downpipe as I'm a do it once kind of guy, and I have to say, I'm impressed with it. Nice thick cast outlet elbow which sounds very refined, no tinny noises. 83mm ID, so minimal step and sport cat and pipe quality are great. Perfect hanger alignment and perfect length to join onto the OE cat back. The only thing that lets it down is the sport cat. It's crap and fails the MOT, but it doesn't bother me as I 'know a guy'
I'm normally not a fan of aftermarket tuning products because there's usually always some crappy flaws, but APR's piece did impress me.