I went with the missus last year and it was my first ever grand prix. Going to go this year as well and might do the same thing (general admission tickets on just the race day)...
In 2008 it tipped it down, 2009 wasn't too bad at all though, quite cold in the morning but in the afternoon the sun came out and it was carnival weather... Really contrasting morning and afternoon...
I did get slightly sunburnt even though it was cloudy for the whole morning...
We brought a couple of comfy deck chairs with us and sat at the Maggots/Becketts complex. Loads of people there but we got through the gates at around 6am so we got a seat near the front.
Was an awesome day, tickets are expensive though, don't think I'd wanna spend on the more expensive grandstand seats.
One of the advantages of general admission is that you can sit virtually anywhere around the circuit and get a lot closer to the cars. There's one part of the track after Copse where the spectators are literally 10 meters away from the car. Insane