I don't know how popular their client is but I have a HD7970 which is not bad at all (enough to run any currently released game on full settings) and when I don't game I let it mine which gives me about $0.5 - $0.75 a day.
An HD7970 can peak at around 200W consumption, so the entire PC is probably consuming around 300W, or about 7kwh per day. I pay $.11 per kWH, so I would be losing money on the deal.
I'm in it for the women tho not the money.