I only buy charms on sale or from Connecting Threads.
They come in handy and some are really nice to have. Ive made several quilts from charms but you need at least 3 packs of the same charms for a lap size. Then you need to match for borders from your stash if the line is no longer available.
It is also nice to use them as a sample before ordering a fabric line since ordering online is not as good as seeing the actual fabric. I like to do this with Connecting Threads since their charms are so reasonable.