Well as for me the reason why i went with zen is because of customer service.
For me in my experience the customer service is better indeed when compared to virgin media's customer service so i give zen that one, but how much better well i don't know but they are uk based and do not read scripts.
Personally in my experience zen is not perfect and indeed they are more expensive, so for me i am eyeing sky broadband if we ever change isp to give them a go.
We had a billing error from zen but zen fixed that otherwise we would have paid over £100 on the bill, when we already had a telephone package so that was fixed.
Apart from that which has put me off a bit from zen, also don't fully believe the reviews but they are an indicator of how good the service is.
Dan.