I've played a Champion to level 16. I've consistently been an anchor for the party in terms of HP and damage (thanks to GWM). However, I have noticed a few things.
1. Remarkable Athlete might as well say, "You may add half your proficiency bonus (rounded up) to your Initiative modifier." Outside of some effects that required Strength checks or Dex checks that I couldn't use Athletics for (which is a surprisingly small number) the ability hasn't done anything. Yeah, it's neat that I can jump farther than an Olympic field athlete, but it's actually never come up. Who cares if I can easily jump 25 feet if I never need to jump more than 10?
2. Increased crit range is decent, but not worth two levels to get to 18-20. I don't think it should be 18-20 at 3rd, but consuming my 15th level subclass ability is really lame.
3. Indomitable is genuinely terrible. I know every Fighter gets this ability, but it's so worthless. By the time you get it at 9th save DCs are all 17+, and you have two types of saving throws: Those you have like +9 or better to, and those you have like +2 or worse to. So you almost always use it on saves you've only got a 25% chance of success on anyways. It never feels good to use this ability. It feels like, "Reroll a save you still can't possibly make. Pray for a natural 20!" or "Waste an ability rolling d20+12 vs DC 17. Nice rolling, chump. I bet you won't need this later, right?" Finally, there are two levels where your only class ability is Indomitable. Quite honestly, this should just be like a Legendary save (auto success 1, 2, or 3 times a day) or like the original design (advantage on all saves).
4. Your only difficult choice is when to Action Surge. You want to use it early in the combat so it actually does something meaningful, but you have to balance the fact that this combat might be easier than next combat (or, worse, there might not be a next combat today at all!). And once you use it, you're stuck on auto-attack. So it's a complete gamble when you should use it, and you feel punished for guessing incorrectly by having nothing interesting to do until you rest. Without feats, the class would be unplayable.
Overall the biggest problem I've had with the Champion is that so many of the abilities are just, "Hey, remember that one ability you sometimes use that you got 5 levels ago? Well, it's a tiny bit better now. Thanks for leveling!" That's not a problem unique to Fighter (e.g., Barbarian Brutal Crit) but it feels like a poor design pattern that hits Fighter harder than other classes. I mean, Rogue doesn't have to consume a level for every die of Sneak Attack.