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Mark,The compression bit is the ticket when routing completely through something with two show faces, especially if they are fragile, like double sided laminated panels or good two sides veneered plywood.This is most common when routing to a template for curved parts. Both faces are being pressed (by the opposing shear angles) towards the panel core as they get cut away. Thus the name "compression bit."Bill

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I use a 1/8" for doing inlay work, but I'm careful to keep the bit moving as I plunge it in, to avoid the "blow out" issue.  As another poster said, using larger-diameter downcuts in a handheld router can be "exciting" in an unpleasant way.

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A down cut spiral would be best for shallow stuff like hinge mortises, but as a practical matter, I don't think it matters that much if your bit is reasonably sharp.

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Spiral upcut will clear chips faster, but you have to worry about tearout.  I prefer spiral downcut when mortising with a router because it gives a cleaner cut.  Trade off speed for quality.

I use an upcut spiral bit for deep mortises (as in mortise and tenons, since the bit will thrown the chips out of the hole.

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Downcut spirals can be downright dangerous in hand held routers. the cutting action forces chips and adjacent material down, or the router up. I have seen routers fly with downcut spirals in heavy cuts. They are best used in an overhead router to help keep work and templates on the table. Dave S

Could someone please help me? When is it appropriate to use an spiral upcut router bit? A downcut spiral? And just what the heck is a “compression” spiral?

Downcuts leave much cleaner edges, but are not self-clearing.  You can't plunge them into the wood unless you are moving them laterally simultaneously, so the chips have some place to go.  If you do plunge a downcut in one place, it will "blow out" a hole larger than the diameter of the bit.

Ah, so my noodle may not be half-baked after all. Thanks for the info all!Lee, thanks for the info on the compression bit.Mark

The general theory is that down cut leaves super clean edges on the cut, upcut clears the material better. Of course, woodworkers being the creative bunch they are, there are many other theories that work for different people.

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My intuition tells me to use the upcut for mortising for tenons, a down dut for mortising hinges or inlays but beyond that…bupkus.

A compression spiral has the spirals machined in an upcut pattern at the bottom of the bit (farthest away from the router) and a downcut machined at the top (closest to the router). This, in theory, is to eliminate the tearout at the top of your workpiece caused by a conventional upcut bit (or at the bottom caused by a conventional downcut bit). I say in theory because I have never tried one myself, but have read about them in the various tool catalogs. Hope that clears things up.