Is the 5.7 x 28mm a good self defense round?  What are your thoughts on this caliber overall?

Depends on what situations you think you are likely to be using the 5.7. The round was not originally designed to be a general purpose munition. In the 1980's, NATO was worried that the Soviet Union was fielding body armor to their troops and especially their special forces Spetsnaz which would operate behind the lines and that meant the NATO rear echelon troops carrying pistols and smgs would not have anything that could defeat body armor. The 5.7x28 and the 4.6x30 were designed for a new class of weapons PDW which was suppose to replace the sidearms and smgs used by NATO troops. This means the purpose of those rounds is to defeat body armor  For "soft" targets, they do no better than the standard pistol caliber rounds.

 I would rather have a .357 or a .44 Mag. Just me.

No.  Incredibly expensive ammo. Better off with a 9mm or .38 and actually shooting it once a month.  It over penetrates. If what you want is POWER .357 or 10mm is vastly superior.  The bullet was NOT DESIGNED for a pistol. It was designed for a personal defense device the size of a submachine gun to be able to penetrate a NATO steel helmet at 300 meters. That's all.