I'm going to review the entire series, rather than only the first book. Hopefully it will help if some are prepared for what's ahead.
Drake and Flint have killed this series for me with their childish sense of humor.
Jokes that were only 'cute' when they first appeared (occasianally) in An Oblique Approach, are downright nauseating when they are FEATURED in the rest of the series. Every character--be they Thracian, Greek, Persian, or Indian--has an identical sense of irony and sarcasm, and Drake and Flint bash you over the head with it like a cataphract wielding a cudgel. Fer cryin' out loud, even the friggin CRYSTAL sports a sense of humor (I use the term loosely) that is identical to every other character in the series!
Half-way through In The Heart Of Darkness I was rolling my eyes at the overly-pithy, self-aware wit that was creeping up more often. By the end of Destiny's Shield, I was groaning audibly.
The characters are all cookie-cutter: You've got a couple of hard-as-nails hookers with hearts of gold; some grizzled old soldiers with unfailing loyalty to their general; some enemy generals that just drip honor (so you know they'll be switching sides before it's all over); and the vile, honorless, and militarily incompetent enemy generals who howl at the guile and cunning of Belisarius, then ignore the counsel of talented underlings who have spotted his traps.
The battles are pretty entertaining, strategically, but they are all routes. There is never a sense of danger, that the battle could go either way but for the brilliant battlefield maneuvering of Belisarius.
All in all, An Oblique Approach was a pretty darn good book. The account of Princess Shakuntala's rescue was particularly well written.