We recently used Sixt on our trip to Florida. Pick up was very easy, though they took forever trying to upsell us on bigger cars, satellite radio, GPS, and whatnot. That was a bit annoying. I actually told her it's no for everything but she said she still had to go through her list. All in all though, we were done in about 15 minutes and in our car.
Drop off was a nightmare. There's no clear drop off line, you basically have to park in an adjacent strip mall and then....who knows? There was nobody there to process the drop off and no signs explaining what to do. So, we walked in the office to ask what the procedure was and they told us there was a guy outside that was going check the car. Problem is, he was nowhere to be found.
There were 4 or 5 other customers waiting with and everyone was equally aggravated. We finally said something to the manager and he tracked the guy down, but we didn't appreciate the added stress.
Sixt is always solid on price, and the car we got was fine so the fundamentals are solid. If the drop off procedure was improved, it would easily be a 4 or 5 star rental agency.