Pros: Designed expressly for this job and does it well.
Cons: can carve an impressive gouge in the frame if you're not careful.
OK, repeat after me: "There should be a special place in Hell for people who paint windows shut." You know, people like the ones who used to own the giant (formerly) white elephant on the prairie, a.k.a. "my house." OK, in the defense of do-it-yourself decorators everywhere, it is darned near impossible to paint a window, inside or out, without painting it shut. So I guess the special place in Hell should be reserved for those who paint windows and then don't bother to unseal them (including my former owners, of course).
If you've ever moved into an older house or apartment and found out you can't open windows that have been sealed shut with a layer or ten of paint, have no fear, 'cause your answer's right here: the Red Devil Windo-Zipper. Where others are reduced to tapping in their all-purpose putty knife all the way around the frame, the Windo-Zipper (for some reason, also known as the Window Zipper) is specially designed for this one job - and it does it darned well. The Zipper's just a flat stainless-steel blade that looks sort of like a demented, sawtoothed heart; a blade that's screwed firmly onto a hardwood handle. The tip is pointed and has "horns" (I did say "demented") that you can drive into the space between sash and frame on any side. Once the Zipper's pierced the paint seal, just slide it along the crack, sawing out the paint and breaking the seal. Do all four sides - inside and out, if you have to - and then you should be able to ease the sash up through its tracks. Don't be like someone I know and keep pounding upwards on the frame until you break the top stile away from the rails...
If used carefully, the Windo-Zipper won't mar the finish of the windows or will mar it only slightly; however if not used carefully it can jump the crack and carve an ugly gouge. Slow and steady should be your watchword.
For just a few bucks you can have fresh air and even an emergency exit where there once was just a glass-covered hole in the wall. If you live in an old house where someone went nuts with the paint, get yourself a Red Devil Windo-Zipper today!