I booked an independent trip to Mexico in Januray 2013 and bought the guide book as a reference guide. I'm a fan of Lonely Planet books, but this one i'm afraid is pretty average.
I'm basing this review on my experience of the Riviera Maya section within the book - to be fair, it does cover the whole of Mexico, but it really falls down on the hotels and restaurants section.
I moved around once there (avoiding Cancun as much as poss!), and can highly recommend Tulum (lovely beaches / sea / maya ruins / tho slightly hippyish!), The Coba ruins (a great trip for an hour or two clycling round the ruins), Chichen Itza (obligatory, impressive, but an hour or two is plenty), Valladolid (lovely colonial town with a traditional mexican feel) and Isla Mujeres (great island feel, fairly touristy mind).
So - as with other Lonely Planet's, it has all the usual good nuggets about the history etc, but there are very few options of hotels and restaurants (and there are loads of good options once you get out there!) - and of the ones that are mentioned, the writers don't really give you a good feel for them - instead describing most things as "ok" or "average." As a result, from reading the book, you don't get an idea of what ones are the best places to stay at or eat - almost like a lack of opinion which was disappointing and you might as well just go online to look at hotel options.
There is a real lack of restaurant options too - and again - there are lots of really good options. In Tulum for instance, pretty much every cabana or hotel has a restaurant dishing up some good grub - good quality, good value, and some lovely places.
A good example is Harry's restaurant in Cancun. The book describes it as serving up good steaks or something non-descript along those lines - so this place wouldn't look out of place in London - vibey, in-house DJ, amazing steaks (better than gaucho!), great service and just a really cool place. Its pricey - especially by Mexican standards - but if the book had given an accurate description, i wouldn't have worn my thongs! Luckily, i chose trousers and a shirt so avoided potential embarrassement!
Another example of the lack of info is on Isla Mujeres. Again, just a few random eateries listed in the book - "a nice option for breakfast or lunch" - but there's couple of really busy streets off the malacon that are full of restaurants, bars and loads of atmosphere. No mention of even the streets in the book!
So - to summarise - if you're looking for a guide book to get a rough feel for a place, choose an itinerary, learn about the history and culture etc then this is fine. But, if you're looking for a book from which to book hotels and choose restaurants etc, then its pretty crap.