Glass has a variable weapon power, but usually extremely weak. I've heard, without proof but with good reasoning, that when you load the game, it finds that glass has no weapon values like shear, impact yield, etc, so it randomly assigns the value. This comes from leftover bits of memory, and can be completely random, but is usually a negative number that rounds up to zero, giving poor weapon results in all cases. This also means that you should be able to close and restart DF (not just save and resume) to re-roll the glass stats, but who knows really.
This bug is supposed to be fixed. Glass should now have a low, but fixed, value.
The basic gist is that glass is about as good as wood, but with slicing goodness. It shouldn't be used for its awesome lethality, but for its ease of production, low resource cost and high value. It can (and will) kill unarmored or lightly armored troops (by hitting unarmored areas) but will not perform well against armor of any real strength. Fro thinning a siege out, it's probably not bad. For dealing with small ambushes, it's probably not too shabby. For brick-wall, nothing-gets-through, hold-the-line, here-we-stand-here-they-die sorts of situations, you don't want glass.
Either way you slice it, glass doesn't slice much at all. The rule for maximum effect is "1 per trap" but glass isn't good. If you follow that advice you might end up with hundreds of tiles long and still get live goblins.
With the bug fixed, you won't get hundreds, but it's not as effective as metals, so you will need more.
So, my advice would be to pack them in tight and make a grinder of it, to cause the goblins to keep re-pathing through the same traps again and again. Marksdwarves with bone bolts can be stationed behind a pit+fortification to shoot down goblins who survive the grinder for too long.
This part, however, I'd keep. I'd also ask about the deployment. Glass traps work well enough for dodge-em traps, due to infinite materials and relatively low lethality. (Do not use glass for danger room traps, however, or there will be blood.)
Please note, all of this is purely anecdotal. Your mileage may vary, consult your doctor before beginning an aspirin regimen, may contain nuts or nut products. This product is not intended to treat or diagnose any condition. Do not taunt Happy Fun Ball.