I'd used Zip Transfers in the past which is why when I booked them to pick me up from Gatwick Airport when I flew back from Montpelier in the spring and the driver was 40 minutes late, I assumed it was a 'one off' and gave them a second chance when we recently flew to Seville.
This time however, they had subcontracted the service to a local taxi service; perfectly pleasant chap, but a dirty taxi is not what I'd paid for and he told us we could have booked him directly for significantly less.
When we returned to Gatwick Airport (I'd booked a 18.10 pick up) the driver wasn't there again. I called the 'help line' and they were totally disinterested. Eventually the driver arrived without a word of apology at 19.25, by which time we were the last ones in the arrival hall. I wasn't offered any apology or refund. Since we'd paid extra to sail through Passport Control because we had to get home for family reasons, we were pretty livid. It would have been quicker (and much cheaper) if we'd taken the train.
We won't use them again. And if you are looking for a reliable, friendly service I suggest you use one of the many other airport car companies too.