Just wondering if those .22 conversion kits are any good. I want a .22 for those rainy days shooting at the indoor range, and for cheap-O plinking. I know Ciener makes a kit and USAF conversion kits are available in the parts section for $120 + $7 S&H. Are they totally reliable or should I just go out a get a a .22 rifle.

Well, a Rem 597 runs $140+tax at Wally World, magazines are $8 each, 10 shots, no hicaps.
Ciener kits are like $140 with a 10 round(for new, used can be had for $100 with a 10 round), 10 round mags are $30, and 30 round mags are $60, IIRC. So, do you want an independant gun with cheap mags (and expensive hi-caps if you get a Ruger), or expensive mags with practice on your -15?
I have both a Colt .22 kit and a Rem 597, and I enjoy shooting the Rem 597 more, mainly due to the magazines being easier to load and the gun having a bolt hold open.  Colt .22 kit mags have a sharp vertical piece that sticks out of the left feed lip and it is murder on my thumbs when loading it, so beware.

Kharn

If you get a .22 kit for your AR15, make sure you have one upper reciever with a slow twist (1:12 or 1:14) that you only use for .22s.  Shooting .22s can lead a barrel, especially fast twist barrels (1:9 or 1:7).  

Having said that, BUY a .22 rifle for $150 and save your AR15 some dignity and unnecessary wear.  That's my two cents.

I think buying a second rifle is the better choice.  For a little over $50 you can buy a Romanian training rifle or for a little over a hundred you can get a Marlin Model 60, if a repeater is what you want.  With that said, I still want a 22 bolt for my AR-15, because it would be nice to have the coolest looking 22 on the range.

BTW, how does the trajectory of a 22LR round fired from a Ciener compare to a 223 round?  Would practising with the 22LR create some bad habits if you ever had to use the 223?

Again, I think a .22 kit could potentially cause problems with an AR15 barrel (especially fast twist barrels), but from what I understand it is possible to zero a rifle with .22 rounds at 25m and have that same zero work with .223.  

In addition, shooting anything in .22 is a great way to work on marksmanship fundamentals, especially for beginner shooters who are not used to recoil.  

Buy a nice .22 rifle, forget the conversion.  That's my opinion, anyway!