Like 90% of Tau units, sadly, they aren't very good. Low volume of shooting, and moderately-expensive points cost means they're just more durable Gun Drones.
However, their loadout is pretty good- if Burst Cannons get a buff in the future, Heavy Gun Drones could be awesome- it just depends how Burst Cannons are modified in the Codex, and how FW Tau units change in the future too.
Ive used them to fill out a "brigade in a box" which is a full brigade in an Orca. On the table they basically filled the same role as gun drones, just taking less seats to do so. I used twin BCs, since my primary marker support comes from elsewhere.