This may be of little value by now given how many years this rule set has been available (8 years I believe). Nevertheless, I played my first two games using the system this summer and I am an absolute covert - so I thought I'd explain why.
I have been a longer aficionado of the Warhammer Ancient Battles (WAB) wargaming rules for what must be about twenty years. It's the set that got be back into Ancient and Medieval wargaming and is responsible for me developing several armies. I am an historical gamer who only gets to play less than 5% of the time I spend in the hobby ... I suppose I am a researcher/builder and painter in truth.
I like simple, elegant rule mechanisms which moves the game along at a good pace and I play exclusively with friends. I don't do competition. I play with rules as a guide as opposed to strict doctrine and rely on gentlemen's agreements as much as the play-tested scripture.
For me, Hail Caesar takes WAB and simplifies it just that little bit further inasmuch as I see it playing the unit rather than the figure. The ranged attack and hand-to-hand combat mechanisms are the same but the numbers if not the frequency of dice rolling is reduced. I like the save mechanisms in both but I suppose Hail Caesar is a logical development of WAB. It's the similarities which make this an easy conversion for me.
Now I'm not one for trends in rules (clearly) and when I find a set I really like I stick with it. I will still play WAB but Hail Caesar is my new default. It's still in print of course but that's not important to me either. I do see myself adopting some house mechanisms to take some of the elements of WAB into how my good mate Grant and I will play Hail Caesar.
The real difference and strength of Hail Caesar (and Black Powder for that matter) is the command and orders system. What it lacks from WAB is the combative character and special attributes of the leaders such as priests, heroes and generals. Grant and I suspect it will be really easy to develop command templates which will allow these fun command figures to issue WAB style challenges to single combat or inspire fear/hatred etc to those they oppose or under their direct influence.
As a fan of both Hail Caesar and Black Powder, it's also prompting me to investigate Pike & Shotte which may prove a very challenging experiment given my development of the WAB based Williamite Warfare blog and rules. I may see myself developing a Williamite variant.