Trotsky's History of the Russian Revolution certainly wouldn't stand up to today's professional standards for academic history. Instead, I think it is better read (along with Trotsky's other historical works like 1905) as a primary source that gives some insight into the way a revolutionary of the time understood the history of his revolutionary moment.

Getting my copy off the shelf, Max Eastman's (the prolific translator) introduction says "We believe that this present work, The History of the Russian Revolution, will take its place in the record of Trotsky's life..." I think this is about the right way to look at it. Understanding the history of the Revolution from Trotsky's perspective as well as getting Trotsky's "profiles" on so many historical figures that are important to the revolution is fascinating.

The history itself is not terrible for what it is. The basic narrative that Trotsky presents is basically reasonable. Nonetheless, I wouldn't recommend it as the book to read if you are just going to pick up a book about the Russian Revolution. It's a bit unwieldy (the copy I have is about 900 pages) and isn't sourced the way history books now are. It was first published in 1932 in three volumes. This is makes it limiting in terms of leading you to other sources that you might want to read. If you want to read a well written history of the Russian Revolution Shiela Fitzpatrick's The Russian Revolution is a better place to start. If you are interested in Trotsky and the Bolsheviks more specifically and want to see how Trotsky treated the history of the Russian Revolution just about 15 years after the fact, then History of the Russian Revolution is worth your time.

If I can add on to this question: how reliable is Trotsky's biography of Lenin?