In my experience the biggest problem is the random quality. I'd certainly never buy a set without looking at them in the flesh first. Other points to look for are that they can be quite heavy compared to a known manufacturer, and they are sometimes not very well balanced as manufactured (which means you end up with loads of wheel balance stickers on the wheels).
All that said, I have a set of new RS4 reps on my S3 at the mo - I bought them for a good price with virtually new tyres. I wasn't looking to buy reps, and I don't think I'd rush to buy any again unless the price was right.
Hope that helps.