I have the mid package and it's about as fast as the line can get (5 to 5.5 mega thingies). Costs £5 a month on top of the satellite package. If you are getting the satellite anyway (or in my case have a sports-obsessed housemate that gets it) then it's prolly the cheapest way of getting broadband that there is.
When the service started there were a few teething troubles at sky's end (it hadn't been running for long) and with the router, but those were fixed long ago. It's been very reliable for the past, oooh, year and a half.
Everything is easy to set up. My only regret is not buying their wireless card. I bought a cheaper 802.11g card instead and the signals weren't quite clear enough for a reliable connection. Going to a l-o-n-g cable connection sorted it.
Tip: If using an old 802.11b card and can't connect it's prolly 'cos they don't support WPA encryption. You can change the encryption to WEP (or turn it off) quite easily. There's a very useful unofficial support site at
www.skyuser.co.uk.