What's the general opinion on all of this?
Well, seeing as you asked . . .
Tornado: popular boats, especially the smaller ones, but in my personal experience
Barnet Marine (the distributors) are some of the least helpful people on the planet!
Ribcraft: good boats (I was especially impressed with the 585), and run by friendly people who actually use RIBs. I think the tubes are ugly though (very angular)
Humber: very competitive pricing but tend to be a bit rough and ready. Very popular with divers. Various hull designs varying from medium to deep V, so make sure you know what you're getting.
Zodiac: well made RIBs, with good quality finish. Mass produced unlike the others. Fit-out/seating/consoles tends to be more suited to gentle leisure use rather than offshore (bench seats, small consoles etc)
Just a few thoughts. I'm sure other people will have their own opinions. They may even tell us!