I have heard a lot of complaints about their customer service in the past, although lately it seems they have gotten better. At least, people seem to be getting their orders fairly quickly lately from what I've heard...
I've been using the Champion harnesses for at least 7-8 years now, for my previous dogs and Bianca. They are my favorite type of seatbelt and are one of the few that is actually strength rated. Personally I won't use a harness that relies on plastic buckles/snaps to keep the dog safe in an accident, because these are likely to break in those type of forced. The Champion harness does have plastic buckles (wider/stronger than regular type) but they do not rely on those for attaching the dog into the car. They have two large metal D-rings which do the job there.
The dog seatbelt brands that are either strength rated or crash tested are: Roadie, Champion, PetBuckle and Snoozer.
I have tried the Roadie seatbelt too but I found it complicated and difficult to put on/off the dog, so I got the Champion instead and have been using that one ever since. Any other brand may not be meant to actually protect your dog in an accident. I remember seeing some "car harnesses" at the pet store which had written on the package something like, "This product is meant to restrain your dog in normal driving conditions." In other words, if there is a sudden stop or an accident don't expect it to stay in one piece!
Here is a page I put together showing the various ways the Champion seatbelt can attach to the car:
http://www.chicagocanine.com/seatbelt/seatbelt.html
(people kept asking me what I meant when I was describing the way I attached it, so I made this page to show how I did it...)