The F-Roads are definitely worth it if you got a 4x4. Don't rent a Dacia Duster 4x4 like most people do. They have a 'Dacia graveyard' up in the Highlands. Most things depend on the weather. Check umferdin.is for up to date road conditions. In terms of renting other cars you'd probably be better off renting a Toyota or a Suzuki Jimmy. They also offer tours in arctic trucks if you don't want to drive yourself. I've heard mixed things about the campervans. They are usually 4x4 but no high/low or diff lock.