I think the Trex 500 is arguably the best heli for starters.  It is not too big, not too small, and is priced right. 

The Trex  450s are too small, and are less stable.  It is kind of hard to determine their orientation, if they drift away from you in flight.

The Trex 600s on the other hand are much larger and more stable than the 450 and the 500.  But their battery costs are just prohibitive to many.

And that makes the 500, the happy medium 

Here is another guy confirming that the Trex 500 may be the perfect beginner heli (with training skids of course!)

http://www.thetrexforums.com/?topic=5398.msg56812#msg56812