Many many people. 
You will find much discussion in the forums. I will put a couple links to thread discussions at the end of this post.
My own experience: undocumented changes
My own experience back in August 2015 was that it worked great at that time. However, between August 2015 and March 2016 there were at least four changes made to the integration that were not announced ahead of time. At least two of those would cause the integration to stop working for a little while. You might have to do something minor, like change the name or even just open the app and close it again.
My problem is that I am quadriparetic (use a wheelchair and have limited Hand control) so I have to pay someone else to do almost all of the maintenance, including just rebooting something. And I really needed the voice control or I couldnāt even turn my television on and off.
NonSmartThings alternatives
Fortunately, over that time two alternative and more stable methods were introduced by Amazon. The first was that the echo IFTTT channel added the trigger method. Different people have different amounts of IFTTT lag ā ā at my house itās a pretty consistent eight seconds. That was fine and I use that method a lot.
Then the free yonomi app came out, and at least for now it works even more quickly than the official IFTTT channel. My experience with it, though, is that it seems to get overloaded around 530 local time, and it doesnāt always work for me between 530 and around eight. I donāt have to do anything to get it working again but it happens often enough that I keep both yonomi and the IFTTT trigger method set up for each task. I try the yonomi, but if it fails at all I just switch to using the IFT TT for a while.
In addition to all of these, there is currently a closed beta going on with echo and harmony, so hopefully they will release their own native integration pretty soon.
Using SmartThings does give you the ability to add schedules and automations
Anyway, all of that means you have lots of people using echo and harmony in several different ways, and many of them are still using it with the SmartThings method. That method does have the advantage that you can automate stuff if you want the news turned on automatically as you come in the front door, for example. Or if you want to turn on a channel for 30 minutes when you go to bed and then turn it off again.
Itās always good to have choices.
Follow up discussion about echo plus harmony plus smartthings should go in the following thread, as weāre getting off topic for this one.