The design of the spool is terrific but it's so easy to refill the spool you're really better off buying some trimmer line in bulk at Home Depot and refilling it yourself.
I could see getting a second one to keep a reloaded one handy on standby, but really, it's no trouble to refill the spool yourself and much cheaper than buying replacements. Can't say enough about the product, it's terrific. I love my Ryobi, I have a gas and an electric one (got tired of dragging the cord around but the electric one still works great).