The charge up has saved more shots than it's cost me by a long shot. It also gives me time to realign the shot to the victims head if needed.
Right now I'm playing a Quarian Infiltrator w/ the Javelin(Barrel and Ammo mods) and the M99-Saber(Barrel and stability mods). I use the Jav for the first few shots of the round then switch to the M99 as the enemy inevitably closes. If I have good positioning on the map I can usually crank out 1-3 salvos of double head shots before going to the M99.
Overall it's a trade off of what you want when it comes to the Jav, widow, and black widow.
The Jav plays better to my strengths and that's why I use it.
Edit: Well jav lost 50% of it's ammo today....but it got a 30% damage buff...WOOWOO.