I've used car2go pretty infrequently, but have found it convenient when you need to do trips around town that aren't full day excursions. Once you get over a few hours it is cheaper to rent a car.
But the thing that has made this service incredibly better is that since March 31st of 2016, you can end your trip parking on side streets instead of having to go back to a Green P lot. This actually makes things way more easy to use and encouraging, as previously when ending my trip I'd have to return the car to a lot which was a good 7-10 minute walk from my house.
I also like that they've started to include 4 door Mercedes into the mix of vehicles. More of these please!
On a customer service front, the one time I had an issue, I contacted car2go and they refunded me the amount of the issue caused right away.
No 5 stars for the service though as some of the Smart cars drive worse than golf carts.