The first book, Hard Contact, is brilliant. The next three books do offer some insight into Mandalorian and Grand Army of the Republic culture, but I can't recommend them unless you have a strong tolerance for pro-Mandalorian/anti-Jedi viewpoints. The plots cease to have action and galactic significance, and they become more like soap operas about a warrior cult.
I'm halfway through 501st. So far, not much has happened except for Skirata and his gang showing what all-around great people they are, Palpatine giving evil orders, and more bashing of Force-users not allied with the Mandalorians. It's not quite as terrible as Revelation, but it still makes Planet of Twilight seem worthy of a Hugo by comparison.