Creme de cassis is a darkly colored liqueur from France made with blackcurrants. It has been produced in it's present form for almost 200 years.
It has a striking, deep flavor of berry fruit on one level but also with a lot of structure. A little will go a long way in a cocktail.
The usual cheaper brands such as DeKuyper and Leroux are nothing like the real thing from France. This is one liqueur where it pays to get the real thing. This is especially so because the two most popular cocktails made with Creme de Cassis pair it with only a single ingredient - white wine for a Kir and Champagne for a Kir Royale. So your Creme de Cassis is really on display and it needs to be good.
Fortunately it is easier to obtain French Creme de Cassis than it used to be. I use L'Heritier-Guyot and I love the stuff. Inky in color, with a sour layer that balances the sweetness nicely. The sourness makes it very interesting and is a result of the acidity of blackcurrants. It's a bit of a haunting flavor - similar to the nutty bitterness in Maraschino (not in the way it actually tastes but in the way it makes it's presence felt).
One indicator of a better quality product is if the liqueur has "Creme de Cassis de Dijon" on the label which means the blackcurrants were farmed in Dijon, France. Consider this a little step up from "Creme de Cassis" like Beaujolais-Villages versus normal Beaujolais. Another clue that you have an authentic Creme de Cassis is the viscosity - there should be a slight thickness with good legs in a glass (clings to the sides of the glass after swirling). You can spot this by turning a bottle but it is a lot easier in a glass.
Creme de Cassis liqueur will go bad quicker due to the huge amount of Vitamin C in the blackcurrants. You will want to store an opened bottle in the refrigerator and decant into a smaller container when needed to prevent oxidation.
There is no proper substitute for Creme de Cassis as it has a very unique flavor. If you have anything made from red currants you can start there. But in a pinch your best bet is to look to a cousin of the blackcurrant, the raspberry. Chambord is your first choice here with other raspberry liqueurs being merely serviceable.
If you have found a better substitute please let us know.