I'm honestly still pretty excited about knights. When it all comes down to it, this is a game about cool little plastic dudes. The knights are really cool plastic dudes. It's a beatiful model that has me excited about painting for the first time in a rather long time.
Furthermore, if you ignore the chainsword for a moment, I really think it's not that bad. Sure, it's a superheavy walker. If you think about it, armor 13 with a 4+ invul is equivalent (approximately, obviously) to something that's toughness 9 with 4+ armor (that you can't ignore.... but vehicles don't normally get armor saves). It has 6 "wounds" which can potentially be removed 3 at a time. It's immune to instant death. Lastly, it carries a weapons payload that is far less devastating than certain wraithknight/riptide builds.
Now the Strength D chainsword is an interesting issue. Yes, without question, if it gets into close combat with something, it'll kill it. Yes, it moves like jump infantry. There are a lot of units that are tough in close combat that move quickly. How do you normally deal with those? Running away is a valid strategy... as is tarpitting (preferably with a fearless mob of pitifully cheap dudes, spaced far enough apart that it'll take a while to engage all of them, thus saving you from stomp).
Yes, 5 of these in an army is going to be silly. I don't think it'll be a particularly good army once the meta catches up to it. I think people will more likely bring one or two.
Last but not least: If someone asks if you want to play 40k, you are allowed to ask what they're playing and decide whether you want to play. If a game isn't going to be fun, don't play it.
Tournaments are another issue, but I seriously doubt someone will bring 5 knights to a tournament. Remember those silly Necon Croissant lists? 10 Paladins and Draigo? Eleventy F***ing Riptides and Broadsides? There will always be the buid du'jour, it will always be unbalanced as hell, and they will always go away at some point, when people figure out that they aren't actually good in general.