I've yet to find a good one that I can see clearly outdoors on a bright day. The new IPAD display might be one of the better ones, but you can not simply hook your camera to it, you will need a card reader and adapter, or perhaps a eyefi card will connect?
Drivers for the 7D will need to be installed in order to view images on the card while installed in your camera. There are windows and mac drivers, which limits you to a windows or mac laptop, or a windows tablet.
Things might get better when the new microsoft Pro tablet arrives later this year, but it will cost $1K or more.