First, although the physical product is in two books now (over 700 pages split into a) char-gen and powers; and b) play mechanics and GM stuff), I'll be straight-forward and say there is nothing on the character sheet that a Champions player ... even a player who has Champions I, II, and II from back in the 80's, would be unfamiliar with. The game has changed more than for any of the previous editions, but this is not a radically new game (like Champions: The New Millenium, which introduced us to Fuzion).
That being said there are some differences, but on first read most of those are in character creation, not actual play. Thus, if you wanted to use Mechanon, as printed in the back of the various rulebooks, he should work just fine, although some of his point costs might not be an accurate reflection of the new rules.
Likewise, all of the 5th edition "Ultimate X" books are still compatible .. and there are currently no plans to print 6th ed versions of them all.
What has changed? Well, here are some of the changes I've spotted:
Movement is in metres (meters if you prefer), not hexes. You could still use hexes, but they're not the assumed condition.
A dozen skills and a dozen powers have been altered. For instance, since Armour and Force Field were just the same actual power with different modifiers, they're now not separate powers ... but you can still have armour and force fields.
Power framework "Elemental" is gone, replaced by the Unified Power (-1/4) limitation.
The advantage/limitation fractions charts are back! ( :) I missed those!)
Characters require fewer points in Disads (now called Complications) ... so you now take fewer Disads, concentrating on the ones most important to your character.
There are optional rules for Heroic Action Points ... a pool of points that you can use to increase your rolls, and other optional rules to remove the character cost reduction from Complications (the GM just lets the players have the maximum for the campaign) and instead have Complications that occur during play give the character more Heroic Action Points.
In summary. I'd like to say that you are still rolling 3d6 and hoping to roll under your skill value ... this is still the Hero system. But it does represent the largest revision to date.