Haven't used my Gargantua yet, but the 150 has been my light primary of choice for the past two years. To me, the attributes that make it a great soaker are pump volume and trigger speed/responsiveness. You can fire off a flurry of tap shots in a couple seconds, then pump once and you're back up to full pressure. You can hold back most CPS with that thing. Just keep firing. However I've never seen anyone actually pump while shooting; that would mess up your accuracy.
As an aside, the XP 75 has an even better trigger and a similar-sized pump, but it's PR and the reservoir tends to blow off, haha. That said, it doesn't seem noticeably weaker than the 150. I'd actually be quite interested to see how competitive the 75 is. I don't remember anyone ever using one. It could very well end up being a gun that's better than it's stats suggest. However I would still take the 150 just because I don't like PR guns.
I would say another gun that's better in practice than on paper is the Vanquisher, for a rather interesting reason that my brother pointed out. It has pretty good range (I got just under 40'), and just plain stupid field life, but it lacks the thick, explosive streams of contemporary medium primaries. But it can still do well in that class, and here's why: firing it makes almost no noise. Small CPS guns make a *PSHT* sound when they fire, which has saved me from being hit a couple times. Meanwhile, Vanquisher streams just come out of nowhere. If your opponent turns away, you can lob a shot at them and they won't hear it coming.
There are also guns that do worse than their stats suggest. In my experience, the Colossus is one such blaster. Compared to other light primaries, it's got great range, and no obvious weaknesses in the statsheet. But the stream breaks up quite a bit, making hitting anything a challenge. At range, you're basically just sprinkling your opponent.
Ultimately, stats will never tell the whole story. Just like where certain players may not have the greatest kill/death ratio, but are an invaluable addition to the team regardless, due to their tactical knowledge, chemistry with other members, etc. Another example is Pokemon, where a 'mon might have mediocre stats, but becomes popular due to its ability or movepool or what have you.