I've used SailCare several times and I've found them to be excellent. As Broadreach said, I've only had them clean my dacron sails so I don't know about mylar. The first time I tried them I had to call them when I received my main back 'cause I thought they shipped me somebody else's brand new sail! The process does re-calendar them to an extent and, as pointed out earlier, the cleaning quote won't include repairs. They will, however, give you a quote for any repairs recommended. One time they pointed out that some stitching was shot on the sun-cover of the jib and some batten pockets had to be repaired. I authorized it and got back a perfect set of sails, albeit 8 years old...
I'd definitely recommend them if you want to get a bunch more years out of your existing canvas!
Steve