I watched this OAV somewhat late into watching the TV series, which was a mistake.
Unfortunately, in choosing not to influence the main story arc, the script writers are stuck with not pre-empting any story lines from later episodes, and the characters are cliched representations of themselves - Godai and Mitaka constantly competing, Kyoko terminally indecisive and withdrawn, the loon squad picking on Godai at every opportunity.
The 5 for story reflects this. Story-wise, it goes nowhere; so if you're a fan of the usual Takahashi antics and you do want to watch it, I suggest doing so as soon as Mitaka and Oba-chan have made their appearances in the series.
On the other hand, coming from DVD both the animation and sound are excellent.