Is 90% tungsten better than 80%?

Yes for most players. Higher tungsten content makes the barrel slimmer at the same weight, which helps tight grouping. It costs more than 80% tungsten.

What weights does the Valhalla come in?

Winmau makes the Valhalla in a range of steel tip weights, typically from the low 20s to the high 20s in grams. Pick a weight close to what you already throw.

Does it come with stems and flights?

Yes. Winmau ships the Valhalla as a complete set with shafts and flights fitted, so it is ready to throw out of the box.

Is the grip good for front grippers?

It depends. The milled grip sits along the barrel, so mid and rear grippers get the most from it. Front grippers may prefer a smoother nose.

Is it worth it over a cheaper Winmau dart?

If you throw regularly and want a slim, grippy barrel, the 90% tungsten build is a real step up from brass or 80% sets. Casual players may not notice the difference.