The Champion class is NOT a good main tank. If you compare a Champion with a Guardian or a Warden you'll find one or more deficits. The average Champion will have lesser block-, parry- and evade ratings, lesser critical hit protection, lesser damage reduction, lesser incoming heal, lesser skills for survivability and poor skills to get and gain aggro.
But a Champion can get used as main tank. But that highly depends on the skill, experience and equiptment of the Champion, the instance and the other members of the group. I did quite some main tanks runs with my Champion and i have also a Warden. If a rate the difficulty of instances/encounters with my Champion as main tank:
- Sammath Gûl: not easy but ok
- Halls of Crafting: quite difficult
- Great Barrows (Thadur/Sambrog): hard
- The Blind One in Dar Narbugud: very hard
- Durchest in Barad Guldur: is nearly impossible
and compare if the with the difficulty rating for my Warden:
- Sammath Gûl: easy
- Halls of Crafting: easy
- Great Barrows (Thadur/Sambrog): not easy but ok
- The Blind One in Dar Narbugud: difficulr
- Durchest in Barad Guldur: hard
It obvious that tanking with a tank class is just much easier.