Agree about the sim plus wifi version being money well spent. My ipad2 has been an absolute delight to have so far, even with a few frustrations here and there, like the freezing of Mail on an ad hoc basis!
I quickly worked out it was cheaper to buy a few of the 3 ipad data sims from amazon or ebay and keep them handy and swop out when needed rather than top up. At £10 per 1GB top up from 3, £12 for a 3GB data sim seemed a much better deal to me! I weighed up the cost of a contract and decided that PAYG would def be cheaper in the long run. 3 have seemed to have a decent signal in the Midlands so far, my iphone is on O2 and thats been ok too, but the 3 ipad sims seemed to be the most popular when i bought my ipad and have worked seamlessly. Now if it were just a tad easier to get the sim slot open each time a sim is changed, that would be a bonus!