In addition to the things others have pointed out -- the inaccuracies in the maps and screenshots -- I find the guide hard to use. For example, the map for all the hidden horseshoes in Las Venturas is about 100 pages away from the screenshots of all the locations. The Vice City guide was a great guide, very high on usability. You'll spend a lot of time flipping back and forth with this one. The typeface is really tiny, and the mission descriptions tend to be very simplistic (at least the VC guide gave you some actual strategy tips; much less so, I think, with this guide).
It's big and slick and pretty to leaf through... but that can't justify its cost. You will find less than 50% of what the game has to offer without it, so you're stuck with buying it. Brady should publish a second edition with corrections, or at least publish online corrected versions of the maps and an errata page.