I have and am using 6th gen Time Capsule. I see no reason to upgrade as of right now. It works well and for the amount of devices that I have on wireless, it does a great job. I also only have Soectrum 100x10 internet and it is plenty powerful to service all of the internet traffic we have. The only downside is not getting a clear picture on how much longer it will be supported for security updates.
My 6G Airport Extreme is still working great and have no plans to replace it until there is some kind of issue. That said, wouldn't recommend it for those who are in the market for a new router. Given Apple's exit from the market and the march of technology, there are no doubt many better choices today.