I would say it can be a good thing if the tyres and cars are evenly matched, bad thing if not. Last year, Bridgestone had the best tyre and Ferrari had the best car. Result? Annihilation of the rest of the grid. The tyre war ruined last season.
This year, Michelin and Bridgestone seem more evenly matched, and the new Ferrari doesn't seem as good as last year's car. We've also had 5 different winners from 7 races, so at the moment, the tyre war has been a good thing this season.
So, is a tyre war good or bad generally and why?