I love the look of the 710; it's got a very friendly shape and looks colorful and fun. The design and color choices look very Nokia. I really hope colorful battery covers make it to other classes of phones.
It's a neat idea and I can imagine some of the people who might like it. My friend sometimes likes to wear bright bold colored shirts and hats and shoes. I don't know how many of the younger crowd likes to do so as well, but these colors along with the Nokia/Monster Purity accessories would make an impact.
I sort of wish there were replaceable exterior parts to phones because, if a phone has a good design, I hate covering it up with a skin or case to try to protect it, especially since skins are often difficult to get on adequately and cases add bulk.