Like mac1911, I'm still not sold on the AR clones - I don't like the lack of compatibility, and I'd like to get more feedback on reliability and durability before I consider one.
The 10/22 is a good option, but no matter how close you get it to looking like an AR, it's still not going to feel like an AR. Even the Nordic receiver doesn't quite get the weight and balance down, and for $200 (plus you still need to buy a stock, grip and forend, not to mention the rifle), it's hardly cost effective.
The benefits of a dedicated upper (over the 'conversion kit' BCG) is that you don't have to worry about .22LR fouling in your .223 barrel, you don't have to re-adjust your zero every time you switch from .223 to .22LR, and a dedicated upper is going to be inherently more accurate because the twist is designed for a .22LR round moving at .22LR speeds.
I REALLY like my Spike Tactical upper. It's virtually identical to a standard M4 upper (I've had friends who didn't even know they were shooting a .22 until they pulled the trigger). It's utterly reliable (I'm still waiting for my first FTF, and I've nearly put 3,000 rounds through it, mostly Federal value pack). The Black Dog mags are fantastic, and they're now making shorter mags for shooting prone (which was my only complaint). And when I'm shooting the .22 upper, I'm shooting MY AR: it's the exact same trigger pull, exact same grip and stock, exact same everything. It's the ideal training tool.