My experience with the glasspack...
If all you can do is fork over $30 for your muffler, they work, and work fine. You will NOT be happy with the tone in the long run. Glasspacks are cheap, somewhat less-restrictive alternatives to a 'decent' muffler.
I have had 3 total in my life. I delt with 2 kinds, a 12" and an 18".
As far as glasspacks go, the shorter the muffler, the louder the tone. I had the 12 on a 1987 Cherokee 6 cyl, and a 1989 Buick (gulp) Century 4 cyl. The Jeep was loud, but lots of pops and vibration, esp comin down from high rpms. No throatyness, almost harsh, like hands clapping if that makes sense.
The Buick, well, its sounds like a Mercury outboard at idle, and someone blowing a big, deep, Kazoo when you put your foot into it.
I had the 18" on another Cherokee, and it was better (quieter). At idle you wont notice much difference from a Flowmaster or Dynomax, but the when your driving, the tone of the Flow, Dyno, etc wins hands down in my opinion. I went with a Flowmaster, made that 4.0 liter sound like a 5.7, throughout the rpm range. HUGE difference.
Now remember, thats just my opinion, you may like the tone, and if you feel like taking a $30 chance, why not!!! I just think you will be pleased in the long run, by taking a step up to the $80-$150 muffler range....
Cheers,
Russ