I really enjoyed this episode. (Caveat: I missed the first few minutes. It's on tape.)
This episode hearkens back to the days of character exploration like "Bart Sells His Soul." It's not up to that standard, but at least they're trying. Bart retrieving Martin's belongings and finding the picture saying "Bart, please be my friend" was poignant. (Although, it wasn't completely clear that the picture came from the box.)
Of course, to move the story along, they couldn't let Lisa go to Chief Wiggum with what she knew, so she had to go through some angst. I think they handled it well. This episode could have been completely Bart-centric, but Lisa really seemed to play a great part in it, and she wasn't annoying or know-it-all this time.
Also, I thought it was neat that Nelson got to play the detective tracking down the Simpson kids, when we would usually expect the roles reversed.
-- R Flowers