Hmmm, I feel compelled to apologise for my second sentence above. I blame trying to carry on a conversation with my wife and mother-in-law whilst typing, and the tea not having kicked in yet. At any rate, I trust it's clear the book sounds exactly what I'm looking for...

Just thought I'd mention I'm about halfway through the fist volume of Marius' Mules and am enjoying it very much. It's a great read with lots of period flavor. Appealing, well-rounded characters set against hugely dramatic events (currently chasing down the Helvetii outside Bibracte) make for a real page-turner. There are times when Mr Turney is a little too eager to use technical Roman jargon, or uses a phrase that sounds, at least to my ears, jarringly modern, but this is only a minor quibble. I may be imagining it, but there are also times the legate Fronto and his companions sound a little like proto Englishmen - one almost expects to hear the occasional "there's a good chap" in the midst of the dialogue - but this actually works pretty well, I think. It sets the Romans apart from the local natives, the civilized from what at least the invaders perceive as hordes of unwashed savages (though Turney makes it clear they are often wrong to think this), so overall the approach doesn't jar needlessly. I'm sure striking a good balance in this area is one of the challenges of writing historical fiction.
Finally, I should admit again to being a rank amateur in Roman history, and so won't catch lapses in the portrayal of culture, military practice or events that might bother some here. But as far as I can tell Mr Turney seems to know his stuff.
Those minor caveats aside, I highly recommend the book to any with an interest in the area. My thanks again to Nervisfr for recommending it!