I also watched it last night.....
My advice would be that if you typically enjoy History Channel shows of this type, then you should give it a chance. Furthermore, if you're unfamiliar with the subject, then you might learn something. Sherman is a controversial figure, and you'll get a little bit on both sides of the argument. However, if you are already familiar with Sherman and his march then you're unlikely to pick up anything too intersting.
Disclaimer: I tend to be less than critical when it comes to these shows. Personally, I'm happy when there is ANY variety on the History Channel and they put something on that's not World War II related
In the History Channel's defense, I'm sure they were on a pretty tight budget and couldn't splurge for thousands of reenactors or computer generated effects. Furthermore, I doubt the guys who played Sherman or Grant were actors...they were probably historian/reenactors, so I won't go bashing their consierably weak acting skills too much. Hey, the producers made an effort....at least the guy who played Kilpatrick had the funky sideburns and Howard had one arm! We'd all love for these shows to be impeccably researched, with oscar-caliber actors, and 50 million dollar budgets for effects, but its just not realistic.
On a side note, it would be good if they could do some more shows like the "Decisive Battles" series that took advantage of game graphics (Rome Total War)....or maybe an 18th or 19th Century battles series like the "20th Century Battles" series that is on History International all the time lately.
By the way, I would recommend the "Dark Ages" show that was on the History Channel recently (if you have "On Demand" on your cable system, you can load it up for free).