I've talked about it before, but I have played a Champion from level 1 up to level 17.
First, a note on how I evaluate classes. Any ability gained after level 15 is not worth considering in evaluating the class, or, at least, is only worth considering as something you will almost never get to use. Capstones get this applied doubly. Simply put, 95% of play is at levels below level 15. It's more or less impossible to undervalue these endgame abilities. They're cool and powerful, but chances are you won't get to play with them, and when you do there's a good chance they just won't matter because another PC has something that's just better. Yeah, Survivor is really, really good, but you're not going to have it very long and by the time you do the Cleric has heal virtually on tap.
I was pretty consistently the best or near best damage dealer in the party. Outside of combat, I was mainly useful for my Strength and my personal ability to remember absurd details that the DM gives out that everybody forgets. The rest of the party consisted of a Cleric, a Rogue, a Rogue/Fighter, a Paladin, and a Wizard. I was only surpassed when the Fighter/Rogue hit with a series of Riposte Sneak Attacks or Sentinel Sneak Attacks, or (sometimes) when the Paladin player went nova. Any combat that went 5 rounds or more featured my PC as the anchor. At lower levels, I blame this on the fact that the Paladin player didn't optimize their to-hit, and the fact that I was the only person with a -5/+10 feat. After level 11, however, it's almost 100% due to Extra Attack (2), which is easily one of the most absurd abilities in the game.
That said, the character gets extremely, extremely boring, and many of the Champion and Fighter abilities are just bad. If I didn't have access to feats, I'd never have made it that far for boredom.
First the good:
Second Wind is great, but it often doesn't feel like it does much, as 1d10 + level is often only marginally better than a healing spell or potion. It's best effect is often just faster healing when resting out of combat. It's good because there's not much to do with bonus actions.
Action Surge was good, but with exactly one use per rest (and often one use per day) and it being the only active offensive ability you have, it's just not compelling before level 5. Figure out when it will have the most impact and use it then. I never used it for anything other than the Attack action, even though I was actually looking forward to getting to do that. I'd rather get more uses per day and have it be less poachable by multiclass characters.
Two Handed Fighting Style is great, and it was fun to reroll damage dice, but it didn't often feel like it did all that much (I know it adds ~1.33 dmg to 2d6).
Champion's Improved Crit is great, especially with especially with Great Weapon Master. It procs more often, and more attacks means more crits.
ASIs are amazing, and bonus ASIs even more so when you have feats, but if you don't have feats this is super, super boring.
Extra Attack (2) is absolutely and without any reservations an absurd ability. You go from keeping up to getting way ahead.
Now the not as good:
Remarkable Athlete is useless. It is for almost all intents and purposes a flat +2 bonus to your initiative modifier, and that's not worth a Martial Archetype feature. There are, essentially, no Constitution checks in the game that a Champion will ever make. Athletics is almost always used in lieu of Str, and many Acrobatics checks can be replaced by Athletics checks, too. It gives a +2 to Stealth, I suppose, but since heavy armor applies Disadvantage that's not particularly good. Jump distance never comes up because module designers essentially don't let it. Chasms are either narrow enough to jump with 15 Str, or they're 30 feet across or more because it's supposed to be a barrier. The ability would feel better if it read, "Starting at 7th level, you can add your proficiency bonus to any Strength, Dexterity, or Constitution ability check you make that doesn’t already use your proficiency bonus. Additionally, you gain proficiency in Athletics and Acrobatics. If you are already proficient in one of those skills, you may add double your proficiency bonus when making a skill check for that skill." I mean, everybody else is getting Expertise now. Might as well spread the love.
Additional Fighting Style is good... ish. The issue is that you're naturally picking the second best fighting style because you've already picked the best. I picked Defense since that's the only one that really does anything for a two-handed weapon user, and it's good, but it's rather plain by itself.
Indomitable is, without a doubt, the most unsatisfying and frustrating ability that I've ever encountered. The vast majority of the time, you'll want to use it to reroll a save that you've already got an 75% chance to fail, so it often feels like it just doesn't do anything. On the rare opportunities that you reroll a save that you're actually proficient with, it's often because you rolled really low on a save that you had an 75% chance to make, so it feels like you wasted it. So, roughly 75% of the time, Indomitable is unsatisfying to use. And you have limited uses per long rest, and level 13's bonus feature is consumed by giving you two uses per long rest. Maybe the Fighter should just pick an ability score and always get advantage on saves for that score, and pick a second ability at 13. Sure, everybody would pick Wis, Con, and Dex, but if always having advantage on everything is too good, maybe this is a good compromise.
Superior Crit is also good, again, especially with Great Weapon Master, but it doesn't feel like it deserves to be a whole level's worth of ability. It feels like the 3rd level ability should just improve itself later on without costing your Martial Archetype feature. +100% more crits is much better than +50%, but by the time you get this you've got so many attacks you're often critting twice a round already.
By the end of the campaign, I felt like I should have taken 1-3 levels of Barbarian. Partially because I'd managed to become a Werebear at level 8 of the campaign (too bad everything in the Underdark had Drowcraft weapons), but mostly because Rage, Unarmored Defense (I had ridiculous stats thanks to the maze engine and the DM allowing duplicate rolls... we had the time travel result twice), Reckless Attack, Danger Sense, and Bear Totem seemed a lot better than most of the things I got between level 11 and 17.
