Are casuality markers worth it.  I would use them to replace dead squads or armies but is it really worth it?

Rather depends on your preference and the rules you use. I use them for Bloody Big Battles to mark 'spent' units.  If they were available for WW1 I'd use them to mark 'supressed' stands in GWSH2. I've also used them in other rulesets to mark simlar states of disarray.

The ones I have (Ancients, C17) get used quite a lot, so on balance, yes, if you have rules which require marking negative unit conditions.

In the Polemos series of rules and others of similar design that don't actually count casualties they are often used as "shaken" or "disordered" markers.

is the ACW box set worth it and if so, has anyone purchased it?  What flag sheet does it have?