I downloaded it and have read several of the stories. They're very much in the style of short stories in between chapters in Total Warfare- not surprising since they're by many of the same authors.
If you like those short stories, you'll like Onslaught, and can't beat the price for a bit of fluff reading.
I would not recommend these narratives to those new to the Battletech universe as much might be lost on newcomers. For those of us whose favorite era is the Clan Invasion, worth the price of admission! O0