When I first opened a bottle of Old Rasputin, the term “Imperial Russian stout” was a new one on me. Since when did the Russians make stout, after all? It turns out that they didn't make it, but they did drink it.
When Catherine II was the empress of Russia, a London brewery called Thrale's specialized in brewing an especially alcoholic stout for export to the Russian imperial court. Thrale's was later taken over by Courage, becoming “Courage Imperial Russian Stout.”
Old Rasputin has no direct connection to either of these historic stouts, but it is a very dark stout with a lot of alcohol in it- 9% alcohol by volume, to be precise- so it can claim to be “in the style of” the old Russian Imperial stouts. It's an unusually strong-tasting interpretation of stout, so if you love that dark, bitter taste so much that even Guinness no longer satisfies you, then Old Rasputin might just be the crazy Russian prophet for you.
Most microbrew stouts range from “disappointing” to “repulsive,” but Old Rasputin is an exception. It has a lot of flavor and character, it's nice and thick, and it has much more of a kick to it than a pint of Guinness. Another factor to consider is that most pubs pour Guinness incorrectly, resulting in an almost bland flavor, so a good glass of Old Rasputin may well be more satisfying than a poor glass of Guinness, even if you're a diehard Guinness loyalist.