We used Happy Limo -- the Excursion was great and the kids LOVED it.
My only complaint is that when the driver was late, no one called to let us know. I had to call to find out what was going on, despite the fact that they had my cell #. He was late because he did a cruise pickup/airport drop off before getting us and the cruise was later than usual clearing customs. He was perhaps 20 mins late -- not a tremendous problem, but I would've appreciated a call, especially since there was plenty of warning that he was behind schedule. They were very apologetic, the driver was otherwise great and the limo spotless.
Also, check out the DIS transportation board for more reviews on the many limo services.
-CC