Straight up, I really like this book. I have seen a few functional grammar books, or books that have "leaned" in that direction, and they have either been very close to Murphy-esque stuff or have been very complex, like Halliday's formidable tome. Lock seems to have found a balance of function and understandable content.
The book is logically organised and represents good chunks of information arranged according to various functions. Noun groups, ("representing things"), various verbs, complex sentences, speech acts and a whole tonne more are there. Each chapter has a decent amount of detail, lots of examples illustrating the point and giving everything a good overall coverage. The bottom line is that there is virtually no strain on the reader in getting their head around it all. Lock is very succinct and wastes no paper or words.
All in all, Graham Lock's "Functional Grammar" is a great resource for teachers, and one that should be counted on the bookselves of anyone wanting a good functional explanation of grammatical aspects of English. As mentioned, I REALLY like this book and found it very useful.