People seem to get that impression, because its an instant 300/550/800 damage (depending upon level, reduced by sheild mitigation), and it only has a 6 second cooldown!  So its easy to exhaust your mana in 12 seconds!  Especially after jumping into a system, already reduced mana by 100.

The gun's usage of 75 Antimatter is not out of line, compared to other ships abilities.  In fact, Flak Burst has a higher Antimatter cost (-100) with each use!  Much like many TEC characteristics (and unlike Advent), its a simple, straightforward, easy to understand ability.

Keep in mind structures don't have mitigation.  A couple Kol railgun shots can do tremendous damage to structures, which is handy when trying to take out a repair bay or frigate factory.  I almost never use the railgun on ships.