This is one of my favourite episodes of the series. I really like it in that it focuses on Joanie for once (the first real time in the series).
From the 'ugly duckling' little girl who was always being sent upstairs in the earliest episodes, she has developed into the likeable teenager we see here. Erin Moran plays the part perfectly, and I would rank this as one of her best episodes.
The climatic scene at Arnold's, where Potsie awaits the arrival of his anonymous admirer, is really well played, and shows that even away from the series leads there can be some excellent stories.
One of the best.