The Loch-n-Load is looked at by many to be Demoman's worst primary, and while in some scenarios that's true, there's one area where it's usable to its fullest.
Before that though, let's look at what it offers. First, let's look at the extra damage to buildings. That seems great and all, but stickies can destroy buildings just as easily, so on a stock Demo loadout, this stat is pretty negligible. Second, there's the longer travel time granted by the faster projectiles. On stock demo, the grenades are usually your close to mid-range option, with stickies being your mid to long-range option. Making your grenades able to fill all 3 of those options is useful, but you can do better at long ranged combat with stickies more often than not. Finally, there's the faster pills. This is useful for follow-up damage, and is arguably the only thing this gun has going for it when it comes to stock Demo in my opinion. But with the downsides of no rollers, a smaller blast radius, and 1 less pipe, this launcher is almost a direct downgrade to the stock launcher, meaning that you'd be better off with any of the other 3 launcher options.
So where is this gun viable? Usually it's just used as a crutch for people who can't aim pills, so what possible situation could you be in to make this better than the other 3 launchers? Well, in my opinion, Hybrid-Knight is the way to go with this launcher. Let's look at the stats again, but in the context that you have opted to use a shield in place of a stickybomb launcher. Extra damage to buildings? That is amazing to have, since sentries shut down a Demoknight otherwise. And since you can now 2-shot a level 3 sentry, you can kill a nest and then pick off the Engineer or anyone else using the dispenser to heal, which if you're using a sword like the Half-Zatoichi or the Eyelander will let you leave the fray with more health than usual (overheal for the Half-Zatoichi and a higher overall health with the Eyelander). Now, you may argue that any launcher can take out a sentry base (save for the Loose Cannon) easily, but that's where the second upside comes in. You can outrange the sentry thanks to the longer travel distance, letting you safely pick it off. With other launchers, you'll often have to quickly enter then exit the sentry's range before being able to actually hit it.
But this is only with sentries. Let's talk about the other bonuses this launcher offers for Hybrid-Knight. The longer range also lets you comfortably be able to deal with opponents out of the range of your charge. Normally if you had stickies, this long range wouldn't be an issue, as you can charge a shot with the launcher, but with this, you can take out someone shooting you from a distance that you normally couldn't reach. Take pl_frontier's last point for example. On defense, if you hide out by the little shack in the corner, you can comfortably take out any enemies hiding on the cliff or on the other side of the building. This can let you take out a push from the safety of the shack, and then charge inwith your shield once the powerhouses are either retreating or dead to take out the rest. Considering that this location is a common place for a BLU dispenser, it is often that it'll end up housing a potential push.
This gun also has very good synergy with a shield. Say you miss a melee swing and end up sending your opponent flying backwards. This is where the faster projectile speed comes into play. You can quickly switch back to your primary and finish them off in 1 to 2 good shots. The same goes for if your charge is empty and an enemy is retreating. Your fast pills and their long travel distance can finish off that pesky scout that you almost had but barely missed with your melee.
Based off of what I've observed from using the Loch-n-Load for a while now, it's my current go-to choice for a primary weapon on my main loadout, which also uses the Tide Turner and the Bottle. Sure you lose out on the area denial of the other launchers, or the mobility offered by the Iron Bomber or the Loose Cannon, but this gun's synergy with Hybrid-Knight is nothing to be underestimated, which is why I use it.
Hopefully this can change your mind on this weapon normally used as a crutch, or at least give you a new perspective on how to use it. I've been having a lot of fun with it in my main Demo loadout, being a Hybrid-Knight, and it's capable of doing some serious damage.
tl;dr: The Loch-n-Load is probably the worst choice for stock Demoman, but works very well on a Hybrid-Knight loadout.