I've been spending some time doing research on RC electric helicopers, and thought I'd share the highlights.
I'll do this by describing the two that I own and the one that I'm considering getting.
First, there's the Picoo Z.
The CP is a "real helicopter", while the CX is a "easy-to-fly toy".
Or at least that's what I've been told...
There are two big differences from the CX (AFAICT).
The biggest one is the design of the 'copter. The CP has a single main rotor and a tail rotor, and therefore doesn't exhibit the bad behavior when spinning around.
The second is that the CX is designed to be much more agile, so it's harder to fly.
Neil spoke of not being able to replace batteries. I replaced the battery on my daughter's air hog last weekend, and it works fine now. The Picoo Z battery is a 50mAH battery - this looks like a nice replacement. One of the disadvantages of the low-end RC models is that they charge their batteries too fast (so that it's more convenient). In general, you want a charge that takes about an hour to complete, but the Picoo charges in about a third of that (or quicker), which cuts down on the battery life substantially.
There are directions here on how to open the body, and then it's simply a matter of soldering the new battery. Make sure you get the polarity correct.