Grocery-bag panniers would probably be a better investment... I tried tooling around with one of these tricycles a couple of weeks ago when we were helping out with an event at the local bike rental place. It just felt wierd, and wobbly...
A good set of panniers will hold about as much and the bike will likely be easier for you.
And yep... it's 15 miles from my little abode to the nearest grocery store of any sort.
Tom