If you have free Heavy slots, I would say Kannons are probably the most cost effective unit in the codex. For 69 points you have 3 short range missile launchers at BS3 with 3 re-rolls. In a codex with very little ranged anti tank that can scratch AV14, these guys are gold.
While I eventually had to drop them to free up heavy slots, while they were there they did great things. Just be careful to keep your fire lanes open, it can be tricky with just 36" range. Also, understand that anything that aims to kill them will probably succeed, so burn those re-rolls early.