I've not used their mailer service, but I have used their "in-store" E6 processing where they send it off to a lab for processing. They charged me £6 for process and mount which seemed reasonable but it takes at the minimum 2 weeks! They tell you it will take 2 weeks, but I kept going back and asking if it was in and about 3 and half weeks later they finally came in.
The processing quality was O.K, there were some marks etc on the slides (which I have never got since as I started using a pro lab), but unlike everywhere else they were just mounted slides in a plastic sleeves, no box, which makes them a pain to get in and out (I subsequently brought 10 Fuji slide boxes before they stopped making them). I did use them again to save some money after I lost the process paid mailer for a Sensia film and I got the same quality/time pretty much.
Their mailer service might be faster/better, its not exactly too much to lose if it isn't.