My super-cheap Harbor Freight jigsaw used to cut horribly crookedly and non-squarely. The recommendation I was hearing was to get a circular saw, but I was scared of kickback. So I decided to try a good blade in the jigsaw.
I ordered a Bosch Progressor sampler pack from ebay, and loaded a U234X blade in the jigsaw. While I was at it, I noticed the jigsaw's shoe wasn't square and fixed it. I set medium speed, and turned off the orbital feature, and made a sample cut of 1/2" baltic birch ply against a guide. It cut very slowly, but once it was done the cut was quite straight and smooth. And only a touch of sanding would be needed. The squareness was off by about 0.3 degrees. (Maybe the shoe isn't quite square enough?)
All in all, it's good enough for me. No need for using a router or circular saw.
The U234X (T234X is the T-shank version) blade is cool. It's 0.05" thick and has two alternating rows of teeth, one against each cutting surface, which smooth both sides of the kerf.