I can confirm all the above.
Comet basically have a Sky microsite in-store. I went in and arranged it the first day they were doing it. All went very smoothly.
Watch it though, they'll automatically sign you up for multiroom unless you say it's to REPLACE your old box on the Comet sign-up. Even though (and it's JUST been installed at mine) they'll leave the old box in case of future multiroom use.
Sky Talk is compulsory for 1 month, but the standard package is free anyway, all you need to do is to continue to pay your BT line rental, so Virgin etc customers, you'll need to switch to BT to use this deal.
It doesn't affect any broadband packages you have setup, although you can take this out also.
I've also paid my £25 to make sure my box isn't connected to the phone line (as i don't want yet another wire going round the house).
All in all, very good service, plus a free HDMI lead!