I have had the K100 since 2016, and I HIGHLY recommend it. Although I love flying RC helicopters, I kinda suck at it. So what I needed was a 6-axis CP heli that had a self-correcting angle mode but that also allowed me to change into full-control acro mode and operating in CP mode with 100% head speed...BUT I needed one that would be inexpensive and easy to repair, and MOST IMPORTANTLY it had to be built such that my endless string of "ooops" would be UNLIKELY to damage it in the first place. THAT IS the XK K100.
I've had an Esky Honeybee CP3, a Hisky HCP100S, and an XK K110. The Honeybee was a joke. Somewhat capable flier, but if you so much as looked at it wrong it busted. Very fragile. The HCP100S was an incredible heli capable of things I still won't know how to do 10 years from now, but it had reliability issues and was not particularly robust in crashes...not to mention that the immense power and headspeed made things happen very quickly and made it more likely to be damaged in a crash. The K110 is a good bird, not too dissimilar from the K100, but in my experience the K100 is the MOST durable CP I have personally owned, but also the most durable CP I have seen anyone own (you know, looking at all kinds of reviews and videos and such).
And just by the shear fact that the K100 is still for sale 6 years (give or take) after it was introduced, in a hobby where things have been evolving constantly and every few months there's something new, that alone speaks volumes as to how popular they have been...and for GREAT reason.
I have crashed mine over and over countless times since I bought it in 2016 (actually I also bought a second one in 2017 because I loved it so much and wanted to make sure I had a backup), and they both still fly great. Now to be clear, when I say they crash well and don't damage easily, I'm talking about crashing on a grassy area...not asphalt or telephone poles or running into buildings. Can't speak to that...I stay away from those things. But I have dropped and driven it into the grassy ground countless times and have very rarely had to repair anything...and the repairs aren't overly complex.
My brother also has had one since 2016 and has had exactly the same opinion and crash results (lack of damage) that I have had.
The Achille's heel I suppose is the brushed motor that does wear out after however many hours of run time. But the motors are like $5-$8 or something and can be replaced really easily in just a few minutes. And you might think "why not just bump up a little bit to the K110 that is so similar but also brushless?" and my answer is simply that the K100 flies better and resists damage better, and so to me it's a better choice to stick with the K100.
Disclaimer: Since I haven't bought a new heli in years, I cannot comment on anything newer that's currently available. There might be amazing birds for the same or less money. Look around. Read others' suggestions and experiences.
Good luck!