I picked up a copy of these rules at Vapnartak in February, and having had them a month I've had a good read through them, so I'm going to give my views. Keep in mind what you paid for this review (nothing) and take it in that spirit if it doesn't agree with your views!
It's a Osprey imprint, so it doesn't take up a lot of space on the shelf, and the production values are pretty good, with plenty of nice colour artwork. It's also inexpensive. This is no bad thing. Buying a set of rules for thirty quid and finding you actually don't like them is not the sort of mistake most of us can make too often. The author, Dan Mersey, enjoys a high (and entirely deserved) reputation as an author in the Arthurian field as both a historian and a wargame writer.
Dux Bellorum looks back to some of the older rule sets, in my opinion. The game is set up around bases of figures, with 3-5 figures per base. The precise number of bases in an army varies. An army has a set cost in army points, and each troop type has a cost per base. Hence you can field a small army of high cost troops or a large army of cheap troops as you wish. On average, an army will shake out at about 40-50 figures, which is decently affordable, both in money terms and in how long it takes to paint up and put on the table.
The number of troop types is manageably small, but sufficient to allow a
significant difference in character and composition between armies that
might otherwise be rather similar. Each base is intended to represent
about 50 men, so the rules are obviously intended to represent the
larger battles of the era, rather than small skirmishes.
To further add colour and texture, there are advantages (called "Strategems") that can be selected (at a cost in army points), a different list being provided for each army type.
Game set up looks rapid and the movement and combat systems are relatively simple. Both are dependent on using a pool of points to activate, move and re-order bases. All movement is in terms of base widths. These are deliberately not defined in the rules, so there is not compelling need to re-base anything. Provided both players' armies are based the same way you are good to go.
This is quite a neat touch. Many games now use individually based figures (I'm thinking of not just Saga and Dux Brittanarium but WAB too), and an argument has always been that this is more flexible. I agree with this view - an army based for one set of rules can rapidly become obsolete. With a "generic" base width approach, both players can base their armies of individual figures using movement trays and away you go!
Combat losses are scored as a loss of cohesion by units, and once a unit is totally disordered it is removed from the table. This can slowed or remedied by the use of leadership points, but since the pool of these is limited, you may be struck between activating a unit to fight or saving another one!
The book includes a list of armies for the period. These range from specific Late Roman, Welsh and Pictish through more generic "land raider" and "sea raider" lists. With the choices of army composition and Stratagems, this actually allows for more variation than the number of armies might suggest, as do a set of additional rules for the effect of animals, religion and alcohol on the battle.
Overall, it's well worth the money, I would say. To anyone familiar with (e.g.) Impetus, there probably isn't much novelty, but the additional depth put into the time period makes it attractive.