I'd argue that it really depends on WHY you're buying the .22.
If you shoot competitively, having a .22 conversion kit lets you shoot/practice with your regular gun far more cheaply. One trigger pull, same weight, same gun, in effect, with a smaller bullet.
If you're just plinking, a stand-alone .22 is great.
There are some conversion kits that are BETTER than stand-alone guns, but most of them aren't. (The Marvel conversion for 1911s is phenomenally accurate; they advertize 1/2" groups at 50 yards.)
My CZ Kadet Kit was virtually as accurate as my Ruger Target Competition slabside. It was so accurate, in fact, that I traded the Ruger for a Witness Sport Long Slide. After some work the Sport Long Slide has proved to be a superior gun.
Decide WHY you want a .22 and go from there.