I've back packed across Europe a number of times. Multiple 3 month trips, and many shorter ones through 15 countries (some extensively). The Let's Go book is by far the most up to date one that I have seen. Much more up to date over all than it's competitors (Lonely Planet, etc).
That being said, there is a truly massive amount of information in this book, and of course some of it is going to change over time. The nice thing is Let's Go's structure (check out their web site) keeps this book more up to date than others.
On 1 3 month trip, I had both Let's Go, and Lonely Planet Europe. The Let's Go book only had 1 out of date phone number, while the competitor was riddled with them. I can't tell you how many times Lonely Planet users asked to borrow my books to get the correct phone numbers, etc. But it was often.
As an overall hosteling/backpacking guide through Europe, this book is fantastic. However if you would like more information on a -particular- region, or country, then I would recommend Rick Steve's or Lonely Planet's region/country specific books. Their use of local experts really makes a difference.
Being an over all guidebook, it's not going to have everything in it, so I agree with he other posters that the Eastern Europe sections are a little weak. I'd look at some of the other books above. However, the phone numbers, prices, train fares, train routs, events, etc are accurate enough to help you successfully manage your trip!
I'd highly recommend this book over any of the other European guides if you're planning on backpacking, and hosteling around Europe! If you're looking for a more pension/hotel style book, then try Rick Steves.