We've looked into renting one of these ourselves. We've got plenty of boxes of paperwork, seasonal decorations and clothing, and miscellaneous items that could do well being stored away. But it also seems like a pricey habit to put away things we could probably do without. A 5'x5' storage unit at the nearest location would cost $64-98/month (plus a $8 a month fee for $2000 of insurance coverage...they must think they're Ticketmaster).
Perhaps we'd rent one if we could get a few friends to share the space and price, but otherwise, we've just decided to live with less and rid ourselves of unnecessary items (and to use dear old Mom's garage). But for our expecting friends, perhaps a self-storage unit might be worth considering. I'll just have to remind them not to pull a Paris Hilton, forget to pay for it, and then lose the contents to a auction.