Just seeing this now. I bought a Seasucker at the start of the year and have been using it over the last few months. We've a mk8 Civic and a Citroen c4 therefore none of the normal 'strap on the boot' type carriers work with either car because of the double rear window/spoilers on both. I'm also quite particular about the cars, detailing when I can and keeping my bodywork in good condition. Aesthetically I couldn't face a tow bar on either, plus it wasn't a cheap alternative and also if I sell one or both cars in the next year or two so I'd have to pay for another towbar. Roof rails are ugly, noisy and good ones ain't cheap either so... that led me to the Seasucker.
Sure, the first ride my heart was in my mouth. I had the rear view mirrors locked on the bike and kept to around 40mph, getting out and checking the suction every few miles. By the 3rd time of use I was driving normally, upwards of 60mph and loving the fact that when I stopped it literally takes 20 seconds to throw the rack in the boot. It's not for everyone but so far I've found it a fantastic product for what I want. It means I can bring our 2 bikes, the rather big Croozer trailer and all 4 of us in the car on our wee cycling trips. The kids think it's hilarious that the bikes are on the roof.
If anyone does go for one I'd recommend a bungee cord for the pedals so they don't spin and hit the roof and a bottle of Autoglym detailer and a microfibre to keep the surface clean before and after use.