Are you in the uk?
So far I have found if you go with three there connection is not always great but there data plan is unbeatable. You have to go on a network that offers an iPad data plan. I have 10GB of data per month for £15 on a month by month rolling contract. You can go on pay as you go but the data is expensive. I went round all the networks and even on contract most networks offer just 1GB to 3GB in data. I use mine daily for everything from streaming video to basic Internet usage and I found on my 2GB plan I quickly went over the data limit and you have to wait til the month is up before you get it refreshed or pay extra. Now I have plenty, I don't use a pc so this is my main source for the internet.
Go to a three store and ask them to try a sim card in your iPad to test the reception, then get them to check it in your area too. Although my signal has improved now, after 4 replacement sim cards. Might be an isolated issue of mine but three definitely has the best deal.
I'm told iPhone sims won't work for an iPad also. Have not tested it myself.