My girlfriend bought a cheap-but-chic watch (a Peugeot 705) that she likes a lot. Unfortunately, the crystal wasn't glued in place very well, and she suddenly noticed that the hands and face were exposed. We found the crystal, and I used my old standby: SuperGlue. Bad, bad idea!
Take a fool's advice, by the way: you don't want to use CA glue in a visible/transparent setting where the vapor can't escape as it cures. I didn't make the connection, but this is how they "raise" latent prints on all those police-procedural TV shows. Basically, the inside of the crystal is now coated in an acrylic fog, with a partial thumbprint in the top right. There's a fairly clear area in the center, through which I can see that the watch is still running and actually keeping good time.
So now I want to take the crystal back off so I can clean it... but since SuperGlue is fairly tough stuff, I'm having difficulties. I used an X-Acto knife to scrape out the excess from the outside groove, and I've (sparingly) used acetone-based nail polish remover in that groove... but the crystal remains firmly in place.
The watch body is stainless steel, so it's fairly tough; the crystal is... uh, crystal? I don't know, really. Not acrylic, anyway. My main concern is damaging the works and/or the face.
The watch itself probably cost $50 or less, so it's mostly pride on the line, not money... that and domestic tranquility, which is priceless.