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.

Good stuff:

• Inexpensive ($50 ish)
• Easy to fly
• IR control
• You can fly inside your office

Bad stuff:

• 2-axis controller. Basically, you control the speed of the main rotor and the tail rotor. You can hover and spin the tail around, but not move forward or backwards (well, you can a bit in the the turns). If you weight the front a little, you can get it to slowly move forward.
• Eats batteries
• Hard to get away from your 12-year-old daughter

Blade CX2

Good Stuff:

• RTF (that means "Ready to Fly")
• 2.4 GHz digital controller (a nice upgrade over the old-style radios)
• 4-axis controller
• Stable (ie relatively easy to fly (I was going to write "easy to fly", but it's really not)
• Gyro-based heading control. Basically, the electronics automatically attempt to keep the copter pointing in the same direction.
• Counter-rotating dual rotors (ie not tail rotor to break)
• Pretty-red color

Bad Stuff:

• It has heat issues. The electronics and the motors heat up as you work through the charge in a battery, which means that the trim settings change, especially the trim on the yaw control (ie which way the nose is pointing (I'd call this "tail rotor control", but there's no tail rotor (on a real copter, you'd call this control the "anti-torque pedals", but there are no pedals on my controller)).
  Anyway, what it means is that you start with the yaw torque all the way to the right, and then end up with it all the way to the left at the end of the battery.
  The fix is to a) put a heatsink on the motors, b) remove the case on the electronics and c) cut out the top of the canopy so the rotorwash cools the electronics
• Two motors are twice the complexity
• Yaw is controlled by slowing down one of the rotors, which means the copter drops if you spin it around quickly.
• No inverted flying
• Hard to fly when it's breezy

Blade CP

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.

 (Update)

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.