A friend of mine teacher auto repair at the local community college here is what he has to say,
Oooooooooo....I HATE stop leak, ESPECIALLY pellets! IF this is a "common" problem with those engines, I'd ask a dealer if there has been a recall or a TSB (Technical Service Bulletin) issued.
The ONLY stop leak I like comes in a bottle about the size of an aspirin bottle, is clear on the top and gold towards the bottom, and when you shake it up it turns the color of copper. I think it's CRG or CRC, or something like that. I used it several years ago when we drove our Rodeo to Seattle. The engine kept overheating every 100 miles or so as I was leaking coolant around the intake manifold. I babied it all the way to Seattle, had an Isuzu dealer look at it and they claimed it to be the upper intake manifold gasket, 6 hours labor and $700. Well, I went to a Checker and the salesman told me to use this stuff. There has to be absolutely NO coolant in the engine when you put this stuff in. Well, after driving to Seattle, there was no coolant left, so I put it in, topped it off with water and drove all the way back here with NO PROBLEMS AT ALL! When I got home I refilled it with 50/50 coolant and water and drove it another 2 years with never adding any more coolant and it never overheated again.

So, there ya go....hope this helps

And here is a little more found info for you - The problem is the intake manifolds and gaskets. GM started making their intake manifolds out of a plastic material in order to save engine weight and thus save precious fuel. The problem is that the plastic cannot withstand the heat in that part of the engine and as a consequence the material fails, often with catastrophic results.
GM recognizes this is a problem but they are doing noting to help the people who own these vehicles. On May 26, 2004 GM released a Technical Bulletin (TSB) 04-06-01-017 regarding “New Upper Intake Manifold and Gasket Kits”. In it, it says, “In addition some of the gaskets have been updated to a more robust design”. Only “some of the gaskets”, why not “all”?
Then on January 18, 2006 GM issued another TSB concerning this problem. In it, it says,
Condition:
Some owners may comment on excessive engine coolant consumption, or an engine coolant leak near or under the throttle body area of the upper intake manifold.
Cause:
Upper intake manifold composite material may degrade around the EGR stove pipe and could result in an internal or external coolant leak.
Then it goes on to detail the repair procedure.
Now a TSB is not a recall in any way, nor is it a free repair for the owners. A TBS is for known problems and the procedures to repair that problem.
The cost of intake manifold and gasket related repairs can range from $800.00 of a simple intake manifold gasket replacement all the way to, over. $3,500.00 for a complete engine replacement.
General Motors is not inclined to recall these vehicles or even help their customers with the cost of repairs simply because of the vast scope of the problem. It involves millions of V-6 engines from 1995 to the present. The cost to GM to fix this problem would run into the billions. Something financially strapped General Motors simply cannot afford. A class action lawsuit has been filed against General Motors on behalf of GM vehicle owners to recover the cost of repairs to their vehicles.