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....
No they sound nothing like a Flowmaster. A glasspack usually has only one chamber, and has a fiberglass packing that last 6-18 months. The sound is quite low, and also desirable. But this last for a short period of time, then it sounds like a loud john deer tractor. A flowmaster is fully welded, multi chamber usually, giving a true sound. These will outlast your car most likely.
My uncle has a 4.3 in his fullsize and had dual glasspacks put on. They were called twister glasspacks. I think they have a nice low tone to them but for me they are a little loud but for most people they are perfect. I think he has 2.25 pipes with 3 inch tips.
glasspack mufflers are nice(my point of view) I put one on my 95' GMC Jimmy it sounded pretty good. That was a couple of years ago, it still sounds good till this day. I just currently bought one for my blazer, for now, later iam going to change the whole exhaust system. If you want something cheap and just to slap on, then glasspack is the way to go, but remember thats my point of view.
How about for a 02 4.3 Sierra? I was wondering if I should get rid of the muffler completely and replace it with pipe, put a Cherrybomb in place of the muffler (long or short, I dunno), or cut the exhaust just ahead of the muffler and remove the rest of the pipe back??