I use O2 1Gb a month, but tend to use wi-fi a lot so would use nowhere near the 1Gb out and about, luckily on my iPad 1 SIM it rolled over so usually had at least 3Gb to play with. Hope new card on iPad 2 does the same.
I got my iPad from 3 on a 24 month contract. Upfront cost £349 (includes £30 discount for existing 3 customers). £25 per month for 15gb. This suits me as this will replace my home laptop.🙂