I'll chime in with an unbiased opinion on my Bigfoot clamshell. I bought one a couple of months ago and so far so good. I have it on top of my mildly built LX470. I've taken it on one road trip about 1800 miles and it performed quite well. I was concerned at first I cheaped out, but now I'm pretty happy with it.
Pros:
Pretty good build quality.
Cost about a grand cheaper than the real thing.
Stays put at highway speeds.
Warm on cold nights
Comfortable sleeping pad.
Nice finish on shell.
Bigfoot employees super helpful and quick to answer emails.
Cons:
Slow shipping.
Kinda small for two larger people.
Ladder too short for off road vehicles/doesn't attach to tent securely.
All in all I really like it. I'm glad I got it. So far I have added a solar panel to the top and lights inside plus lights for going up and down the ladder. It's quite the setup..
http://www.bigfoottents.com/