I've had exactly the same experience in Bogota, Colombia.
There was a precise cliff-edge over which service levels dropped dramatically, and it came a few months ago when the lowest priced tier, which had nice, big modern cars with really professional drivers who knew the city, was replaced with a tier branded as 'uberX'.
The uberX experience, while still certainly usable, is a very obvious step down from the previous standard. To sum it up, it basically feels less like you're getting an on-demand professional driver/ taxi, and a lot more like you're just flagging a lift from some guy with a car.
The cars themselves are a big step down - much older, smaller, worn out interiors, sometimes even without seatbelts. On numerous occasions the drivers have been far from the professional driver standard. In general they get the job done but they sometimes don't know the roads very well, and I've run the full spectrum of issues like them taking forever to get to the pickup point, often driving past it and circling back round a one-way system etc, having to take ridiculous detours en route because they miss a turn, going via obviously gridlocked routes, asking me for directions along the way to the destination. All of the things you just don't expect to happen with a driver service but you might expect to happen if you asked a friend for a lift. It's not a guaranteed slick service like it previously was, at all.
I'm not sure what happened here but this, along with the Android app being fairly buggy and seeming to get worse over time (takes minutes to lock GPS coordinates, really kills my battery, sometimes completely freezes when you're tracking them coming to pick you up, gives you a locking error message when you've lost your data connection so you can't even see the map, or the license plate of the car, any more) has really downgraded my impression of uber from something which was all-round vastly better than taxis, to a something which is basically on the same level and that I'll just use when it's not that convenient to hail a taxi on the street. And I'm no big fan of taxis.
It's a shame, I used to rave about uber to anyone who'd listen, but if the level of service for the same cost continues to slide like this here and in other cities then I could see myself looking for and switching to competitors much more often where previously I had no real need or desire to.