On a manual I'd say you lose 25% of the benefit as you have to select the gear you want to use to cruise before you turn it on and it won't drop a cog if you get on a steep hill that it can't pull in the gear you have been using on the flat.
It's not a deal-breaker for me when buying a car, but for sure I'd rather have it than not have it, sportscar or not. If you drive the car on the road then IMO it's useful, end of story. Similarly useful but not essential is the warning gong that can be set to trigger at a set speed that my BMW and Mini have. I have it set at 140kmh in the Mini as that's the speed over here that they take your license away and leave you at the roadside unable to drive your car home, so it's worth consciously deciding to exceed that speed rather than doing it inadvertently, especially as Police cars have forward facing radar that can clock you as they travel in the opposite direction
