I would suggest that if they are on sale for over £300 it's a failure to drop prices over time above all.
They are definitely good for mining, but only because they're so much cheaper than the 6 series, and roughly the same speed.
I bought some brand new for £125 each very recently, and sold them for £100 each second hand, so nobody's paying £300+, even if sites are listing them for that amount.
Especially when Bitcoin mining is a profit centred business, starting off with a heavily overpriced card will kill any hope of making money.