I'll admit up front that I know nothing about ARs, but I'm pretty sure I've noticed somewhere that there are .22 rimfire conversion kits that can be used on a "real" AR. Wouldn't that be a better way to practice using an AR with .22lr? I guess that assumes one already has the base AR, which may not be the case. Are the conversion kits expensive, relative to the 15-22? Are they unreliable or finicky? Info/opinions appreciated.
You have 3 options basically
1. Buy a .22 bolt for you AR. Cheapest solution, but you're relying on a retrofit bolt and firing a .22 down a .223 barrel.
2. Buy a .22 upper receiver set. Cheapest ones are $400 or so, on up to $800 or more. You still need sights, optics, accessories to mimic your centerfire as you desire.
3. Buy a .22 rifle. Gets you a second rifle, built for the .22, need to buy sights, optics and accessories to mimic the centerfire.
The 15-22 is option 3, as is the Colt/Umarex Tactical 22. The 15-22 plastic receivers save a bit of weight without making it unrealistically light (5.5lb 15-22 vs. a 6-7lb AR). The gun can be used for new shooters who are shy of the recoil and noise of the .223/5.56, and saves wear to the lower from having additional rounds shot with it.
The Colt/Umarex .22 doesn't have a working bolt catch and is much more expensive.
The 15-22 is a very logical choice! Sig makes a similar rifle, the 522, which would be a perfect choice if you shoot the Sig 55x rifles or want a rifle like an AR that's just a bit different.