There's one discerning factor in whether it's valuable or not. PANIC! is a game that was released just as Sega's VRC was overcome by the ESRB. If your copy is rated "T for Teen by the ESRB" (as mine is), go ahead and open it. It is only worth about $5 in reality anyway.
If it is one of the very few copies with "Rated NC-17 by the VRC," keep it sealed and re-sell it for more. It's the very same game, however the VRC was more eager to toss out 17's at that point because it had done so very rarely in it's history, was about to expire, plus it was highly offended by vaccuum cleaners vomitting.
On the downside, the game is only funny for the first minute or so, until vomitting houswares loses its hilarity, and you visit the same screen for the billionth time.
Let me give you one tip if you DO decide on playing it, though. There are usually at least TWO buttons that advance you to another screen. They make two different sounds. They're hard to describe, but upon hearing them you can definately tell which one means it springs you ahead, and which one drops you back. If you repeat a room that put you back before, don't use the same button, look for another to push you ahead.