Superstar is one of the very few episodes of Buffy (and I mean very, very few) that I don't much like. It's an episode of Buffy of course, so it is better than most things TV (the worst Buffy episode is way better than the best episodes of One Tree Hill or other annoying teen dramas for example) but for a Buffy episode, it's bottom of the barrel.
It felt like the writers were out of ideas and really worn out after the extravaganza that was This Year's Girl/Who Are You? and stuck with their first idea that wasn't awful. The storyline of this episode is particularly weak (and has a few gaping plot holes) and Jane Espenson, a normally brilliant writer (probably the best after Joss himself!) is at her worst here.
I felt like the writers tried to add cheap novelties here to keep this episode from being an utter waste. The opening credits were a really neat idea and gave the world a bit more credibility. Seeing what the world is like when Buffy isn't all powerful is interesting (but was done similarily and so much better in The Wish) but besides that, everything is bad/average.
Jonathon isn't a particularly strong character. He isn't bad but really isn't interesting. He's good when he's a background character (Earshot anyone?) but as the main focus, he doesn't work so well. Danny Strong puts in a good effort, as do the main cast but it is a rather shaky foundation to lay an episode on.
Anya was really the best thing about this episode. She was hilarious as always and injects a bit of life into a stale plot. She gets all the good lines and is the one thing that keeps this episode above 6/10 (usually the lowest I'd give a Buffy or Angel episode, though there are so exceptions).
Superstar is an average episode- poor storyline, average writing and just has "meh" written all over it. It is still an episode of Buffy though, so it earns a reasonable mark from me!