EnglishRose - I'm with you there.
We've been over 6 times this year, and have always rented out cars through DYS. Never had a problem with them at all, and we've even cancelled and re-booked after the prices dropped with no issues.
From reading a few review sites, it seems that most rental companies take a battering. I think some of that is down to the fact that it's human nature to complain when things are bad, but not to praise when things go well!
We always rent through Firefly (via DYS). They've always been polite and helpful at the desks, and they've never pushed their insurance. They open until midnight, which is handy for us as our flights get in after 11pm, when others (Record, for example) are shut, and would charge an extra €40 for their "out of hours" collection. We have our own excess insurance through
http://www.insurance4carhire.com ,and a polite "No gracias" has always been enough. They then put a block on the card, but we've certainly never had anything taken. They'll also check the car over upon return to the airport, and then send an email to confirm that all's OK. In 5 out of the 6 visits we've had an "in the clear" email confirmation before we've got into the departure lounge! We'll always take photos and video of the car before we leave, and do the same after we return it, just in case...and we always hire full-to-full.
On the other side of the coin, some friends of ours went over to our villa in September. They ignored our advice, and chose to go with one other companies (it was either Drivalia or Dickmanns...can't remember which!) which was off-airport. They'd also selected full-to-full, but when they returned the car (with a full tank), they were told that they were still going to be charged €90 for fuel (sounds like the Hertz experience above). They're elderly, and unfortunately they just paid it...but it shows that there certainly ARE unscrupulous companies out there...!
I guess people just have to make the call whether to go with the cheaper options through the likes of DYS, or with the more "local" options (like OpenCar, for example), which in my experience have been very expensive.