One of the people who hit you up for a commission did that? Man.

Done well, fanfic can be an interesting way of challenging writing skills. Sure, original stuff has some benefits, but if one wants to write for an established universe first? I say go for it. Crawl before walking, walk before running. Fanfic is a good way to start out.

(Plus, somedays you just want to work with characters you already know and don't have to create wholecloth.)