There's this saying ...
... it goes: "Surt, sade räven on rönnbären". Translated, that means "Sour, said the fox about the rowanberries".
Of course, the fox never got to taste them, seeing that they're high up in trees -- except in exceptional years, when there's so many berries that the trees bend all the way to the ground.
I find the berries of the rowan (or mountain ash, Sorbus aucuparia) to be more bitter than sour.
There's a good rowanberry year about every three years. This year was one of them.
They're easy to pick in quantity, and fairly easy to use: I clean them (remove most stems, remove bad berries, remove leaves), and throw the cleaned berries into the freezer for 2-3 days to sweeten them up a bit.
Then I put half rowanberries and half apples into the steam juicer, with a little sugar. This juice is extremely tasty, partly because you only get it every three years or so, so you don't get fed up with it. But because I don't add enough sugar to preserve the juice (I don't like sweet things) I usually freeze any juice which I figure would still be around after a week.
You can use the usual recipes for jams and jellies, but rowan needs 1 tablespoon of good booze per half a liter of jam or jelly. That's Cointreau, a good whisky, or a good gin. Don't bother using bad booze: the rowan will bring out the taste of the booze, and if you use bad booze the combination will taste very bad indeed.
You can dilute the strong taste of rowan with apples, if you so wish.
If you omit the alcohol you'll never get rid of your rowanberry jams. If you include the alcohol you won't be able to keep enough of the jam on the shelf of your fridge.
Related entry: Aronia berries