I don't have service with Greenstar, but did meet with them at my house, and wanted to share two items which I found less than awesome. First, their name is misleading - they aren't environmentally friendly, and apparently don't claim to be. They used to use natural gas, but decided that was too expensive.
The other item is their quote: I have a fairly average yard, but was quoted $80 per service (every other week). I see another review here that was getting $60/service, but regardless, $80 seems like a lot for our needs.
Your mileage may vary. For what it's worth, they seem very nice, and were responsive when I made my initial call.