So I finally got around to finishing this the other night.
It's not bad, but certainly not essential for a Pratchett fan. The book has lots of tidbits about life in Victorian London, but not being all that familiar with modern-day London, I suspect many of the place references were lost on me.
The biggest issue is that it's very light on the humour - I think I may have giggled 2 or 3 times in the entire book - and that's mostly what disappointed me. The spinoff material not written by Terry needs to still be funny whilst enriching the worlds of his books. The World of Poo succeeded well there, because Geoffrey's story was amusing and it explored a side of Ankh-Morpork we don't often see. This book explores Dodger's London with some interesting facts, but little in the way of jokes.