First of all, it is important to note that I am not an expert in either Russian or 20th century history. Though I have spent a great deal of time reading about Russia and the Soviet Union over the last couple years, I would still consider myself a novice. Thus, this review is obviously written from the perspective of someone relatively new to the field.
Service's History of Modern Russia covers the entire 20th century, though it is relatively sparse post-1994. His handling of the material seems fair; he strives to cover material from a variety of viewpoints, showing both the positive and negative aspects of his subject matter. He covers Lenin, Stalin, WWII, and Khrushchev - along with all their policies, political maneuvers, and so on - in depth (at least as much as can be expected for 555 pages on 100 years!), and covers the Tsarist period, WWI, Brezhnev, and the early 1980s in the USSR in sufficient detail to easily follow the plot. The mild disparity in the treatment of various events and figures is not a flaw in his work; rather, his writing is directed to the more significant developments, of which there seem to be relatively few in the period of Brezhnev and the early 1980s (pre-Gorbachev); his ~40 pages on Brezhnev supply plenty of detail into the USSR from Khrushchev to the early `80s. In fact, I have found it difficult in general to find material on Brezhnev. For example, there are many biographies of Lenin, Stalin, and Khrushchev, but Amazon.com does not appear to carry even one on Brezhnev!
As a newcomer to the field of Russian history, culture, and language, I found Service's work to be readable, informative, and straightforward in its presentation, even when the narrative bogs down in groups of names doing this or that. His even treatment of the Soviet Union allowed me to see aspects of the USSR, Russia, and communism which I had not previously understood in a new light. I would have appreciated a short glossary for Soviet political positions and USSR governmental positions which simply defined the duties and responsibilities of various jobs (and perhaps their relative political or governmental clout, if such a thing were possible). It is not difficult to get bogged down in the myriad offices mentioned in the book.
I highly recommend this book as an introduction to the Soviet Union's beginnings, rise to power, and eventual decline and implosion. It covers a wealth of material in a short, readable space and is easily navigable. References are extensive, so further study could easily be pursued with Service as an overview and handy starting point.