I saw another thread on this, I believe on page 3, but it didnt answer the main question I have.
What are the drawbacks of these kits, opposed to others?  

It seems like a good way to burn up some ammo, plink around a bit, and get the kids into it a little more, but If it'll foul my barrel up real bad, or have other bad side affects, that ~$175 could buy a lot of 223 ammo.

I realize the dedicated 22lr upper is definitely the better mousetrap, but the price of these is attractive. How do they compare?

I've never used the Ceiner kit (but have the DPMS and M261), but the issue on conversions are the same.

Just clean the barrel and locking lug area really well after shooting .22's and you won't have any problems. I have never see or heard of anyone ruining their barrel ever. I could see that a few thousand rounds on full auto may clog up your gas port or leave lead fouling in the bore, but both can be cleaned out easily.

After shooting thousands of rounds thru my Ceiner I have yat to see any problems at all..... just shoot a mag full of .223 when your done and give it a good cleaning and your all set .....

The gas tube will blow a bunch of powder residue all over the inside of the upper receiver, but wipe it out good when you're done and you're all set.  I've never noticed it getting the locking lugs dirty.

I try to clean the bore before I shoot any .223s in it again.  That way I don't have the .223 bullets grinding the lead into the rifling.  I suppose if you stick to copper-washed .22s that isn't a big deal.  I'd try to stick to copper-washed anyway since it'll clog up the port less.  High-vel bulk like Rem or Fed 550 packs from WM work well.

Don't expect too much in the accuracy department, but you should be able to hit tin cans at 25yds easily.  If you have a light match trigger (<4lbs) it probably won't work with the kit.  I had to put the stock hammer spring back in on my JP.

The only probs I've had with my 22 conversion is reliability problems, as far as failures to eject the spent case.  The 22 LR can't hurt your barrel it can clog up your gas port but one or two 223 rounds down range after clears that up.  I only use my  A1 barrel as it has a 1-12 twist which is a lot closer to the 1-16 normal 22 rifles twist. So the accuracy is better then  1-7 and 1-9 twist barrels.   I also stick with copper wash 22 ammo like value pack federal HP bulk pack from wal mart.  The only other issue I've (seen) not mine is firing pin breakage in the bolt. A Very fun toy.   I'd say get one.  762