I live in Chicago and use Uber and UberX extensively. Uber taxis are regulated taxis that use Uber as an additional source of revenue rather than driving around looking for people. UberX are just stay at home Mom's, or people who have quit their terrible teaching job to make more money working their own hours to drive. Since UberX has looser regulation on insurance and background checks and the like, using UberX is dirt cheap. UberX has been targeted by taxi companies constantly and recently I think something has passed to make it really hard for UberX to maintain it's dirt cheap rates. I use UberX constantly, sometimes a really sweet BMW picks me up, and sometimes it's some crappy Camry, but overall these are just nice people looking for a few extra $$$'s. One of my friends even started driving after work and she already has a great base salary at work.
Uber also gives you referral credit for ref'ing people to use it for the first time, give $10, get $10, I get free rides all the time signing my friends up for it.
There are a few companies I actively tell my friends about because they are awesome, Amazon, Spotify, and now Uber.
*Note, I may not have described Uber/UberX perfectly, that's just my jist of what i've randomly heard from people. Also, I know this sounds really spokes-person-ish, but I live in a weird area in CHI where cab's are not constantly buzzing by, Uber is a freaking godsend. I long for the day Google buys Uber and we can have on demand dirt cheap self driving taxi's for me after the bars.
EDIT: The main draw for me for Uber, is just 3-10 minutes before everyone is ready to hit the bars, I hit a button, and they are on their way, I can walk out of the cab and it pays w/my CC with a preloaded tip + uber fee, I dont have to worry about asshole cab drivers refusing my CC, I can split fares w/my friends on Uber, it's just all ridiculously simple, they also have great customer support. The only thing that sucks is their demand based "Surge Pricing" where prices go up to 1.25x-10x based on demand. I almost paid 10x fares on NYE but I decided to walk home. I have been hit with the surge pricing a few times but didn't mind because with UberX it was still cheaper and a better experience than some random taxi outside of a bar.
EDIT2: There is also a really good feedback system for drivers AND riders alike, everything I could ask for in a cab app is there, I once complained a driver did not take the route I requested, the Uber CSR wrote back and even refunded a part of my fare and said they'd speak w/the driver. I have had drivers cancel on me before (probably cause I was too far away) but apparently this affects their internal Uber rating as well.