I've been reading some of the posts here as well as some reviews. It seems Kroil isn't just an ordinary penetrant. How well does it work to break off carbon deposits?

Still need to use a bore brush. Kroil main use is for those that use molly bullets.

Can I just spray penetrant through the bore and let it soak, then scrub it?
Or is Kroil a top of the line penetrant? I usually use PB blaster.

Personally, I don't think Kroil is worth the money.  It's a fair penetrant and decent carbon remover if allowed to soak.

I'll bet some 10w-30 or some Simple Green would be just as effective - for one-tenth of the price.

MMO will work just as well as Kroil.  I never used the stuff on a non molly shooting rifle.

I get all of my Kroil for free! Its a really good penatrant. I have yet to use anything better as far as breaking free rusty bolts. It also does a decent job softening up carbon on your AR's but products such as Gun Scrubber to work better.

It's commonly used with moly because it's able to remove fouling while leaving the bore coated with moly.  A more aggressive cleaner that removes moly would require pre-treating the bore or firing 2-8 fouling shots to re-establish the moly layer.  This works on high-grade barrels that have been lapped smooth, but not on the typical factory barrel that is rather rough.  I can get the bore clean on my target rifles with SS barrels by running two patches wet with Kroil, followed by two dry patches.  Every 500 rounds or so I use Sweet's and JB to clean out any fouling that may have accumulated and treat the bore with Midway Moly Bore Prep.  Using this regime, the first shot from a cold barrel is up to velocity and in the group (sub-MOA).

I use it mostly for freeing up stuck pin and screws and whatnot. It excels at this IMO. For cleaning I put it on fouled up areas like gas pistons and revolver cylinders and let it sit before wiping or brushing off. When picking up a new mil-surp rifle with an old bore, I like to let Kroil soak in it after a once through cleaning, then follow up with JB Bore paste. Seems to work well at freshening up the barrel.

For general cleaning on an AR-15 though, I don't think it would benefit you much, unless you have a LOT of buildup of fouling somewhere.

I like it just for the smell.  

I've found that Kroil works pretty well on rimfire bores and at removing lead fouling. I use it for a bore cleaner on some of my bolt actions. I don't shoot moly. Kroil works better if its allowed to sit for a while. And yes, it works pretty good on rusted bolts and screws.

Heavy carbon fouling, such as on the tail end of an AR bolt, generally requires a pick or some mechanical action or scrubbing. I havn't found anything, even with extended soaking, that will remove that stuff easily.

For a while I tried FP-10... I've tried Hoppes, Shooters Choice, Butches, Kroil, Ezzox, Blue Wonder...etc and I've always came back to BreakFree CLP for a general cleaner.

Kroil--for light powder fouling and rimfires.
Shooters Choice Copper Remover--for heavy copper fouling
CLP--for general cleaning
Blue Wonder gel-- for stubborn carbon in my pistol barrels. I'll also use this in my rifle barrels from time to time. It will remove copper fouling, but not as agressively as the SC or Sweets 7.62.

Also metroplex, for a quick cleaning or patch-thru, the Otis kits are great-- for a better cleaning, get a quality one-piece cleaning rod from Pro-Shot, Dewey, etc... makes life a lot easier. I like Pro-Shot.

Overcleaning probably does more harm than shooting dirty guns.