Vandoren Reeds are an excellent choice for an intermediate to advanced clarinet player. They have more resistance than the popular Mitchell-Lurie reeds, which allow for good tone but are weaker overall. Vandoren reeds are much stiffer and therefore produce better tone, although they take a little longer to break in. The V12 is a special edition that is softer than normal Vandorens, which can sound fuzzy before they finally loosen up. With V12s, you won't have to wait a week before you get a clear sound. The V12 is also specially designed to work best with narrow mouthpieces.
In the past few years, the cane used by the manufacturer has seemed softer than normal. A box of 4.5 strength reeds, which are normally quite resistant, often contains reeds that play as soft as if they are size 3. Therefore, if you are looking for a reed with much more resistance, perhaps try regular Vandorens first.