I got one as an early Christmas gift and it's a blast.
I've seen a few complicated mods listed on other sites that I didn't bother with,
including adding bearings to the main shaft.
Adding a touch of weight to the front, and the optional larger tail fan improve forward flight characteristics.
(That stuff should be included with the kit)

There's one called the "Toilet bowl mod" that's pretty painless:

Because of the long and flexible main shaft, they're very susceptible to the pendulum (or Toilet Bowl) effect.
You can shorten the main shaft, and the distance between the rotors, fairly simply.
Essentially raising the center of gravity and giving the tail rotor a little more mechanical advantage.

Pop off that little bit of silicone rubber tube that retains the upper rotor on the mast.
Slip the upper fan off the main mast.
There's a sleeve on the main shaft that acts as a spacer between the upper and lower fans.
Remove the sleeve and cut it down to 15 or 16-mm (roughly in half).
The top end of the mast is cut to a "D" shape to drive the upper fan.
put the spacer back on the mast, and with an Xacto knife, bevel the face of the main mast
so that "D" shape continues down the mast to just a hair above the spacer sleeve.
Put the upper fan back on the mast and replace the silicone tube retainer.

You might want to lube all the moving parts while you've got it apart.

Once it's assembled, you'll want to make sure there isn't too much play in the main shaft.
Get at eye level with the bird and raise up the gears by their hubs.
There should be enough play so nothing binds,
but not so much that the drive gears can walk off the pinions.
It's a little more stable, forward flight is improved, and there's fewer boom strikes.

(Thanks to Chevy6600 for this mod)

The only other thing I did was take the spring off the throttle lever in the Tx
(and paint the arrow side of the plug white)
I've gotten more air time with this in 2 days then I have with the TRex in the past 2 months.