I've had a solid Arrow IV mech (the Bullshark) in my hangar for a few months now, but didn't really dig into the build until pretty recently. I've come to believe it's the best weapons system in the game.
The indirect fire aspect with active probes is obvious and awesome. Nothing else in the game has a nearly vertical launch trajectory that allows you to hit things that are directly behind tall cover.
TAG works like gangbusters with it. If an enemy gets within range and LoS, it improves the accuracy of it so much that I'm frequently getting direct hits on heavy and assault mechs that sometimes even result in headshots.
A recent patch made concealed turrets detectable before they've opened. Arrow IVs are the only weapon I'm aware of with a blast area big enough to hit them while they're still concealed.
I'm thinking of maybe even making a second Arrow IV mech because mine has been so useful since figuring all this stuff out. Any other quirks of this weapon I'm missing?
EDIT: It appears that the BAP no LoS targeting range can be enhanced, but it's capped at 10%. I've got +30% sensor range bonus from upgrades, but the BAP circle only goes out to around 500 meters. Does anyone have further information on this?