Is sumo cheating? This is a question that every lifter and gym-goer has heard a thousand times. Many give a tongue-in-cheek answer or voraciously defend their side. Famously, according to Chris Bumstead: yes, it is cheating. However, this isn't the right question. Unless sumo deadlift is specifically banned by your powerlifting federation or competition committee, it's legal. What should be asked is if sumo deadlift is easier.
Is Sumo Deadlift Easier?
When considering if sumo deadlift is easier than conventional deadlift, there are a couple roads to take. First, there's the strictly scientific route. Second, there are various biomechanical advantages and disadvantages to consider.
A Physics Viewpoint
A basic understanding of physics shows that less "work" is required to complete a sumo deadlift:
Work = Force x Displacement
If you remember high school physics, you know that Force = Mass x Acceleration. You might also remember that displacement is simply the change in position of an object (measured in length). Therefore, we get:
Work = (Mass x Acceleration) x Length
We can assume that our force required to lift a barbell is the same whether you use a conventional or sumo stance because its mass and the required acceleration to move it are the same. Since the length the bar travels is less for a sumo deadlift, less work is required to complete the lift. One study comparing sumo and conventional deadlift concluded that, "Vertical bar distance, mechanical work, and predicted energy expenditure were approximately 25-40% greater in the conventional group." 
A Biomechanical Viewpoint
Generally speaking, there are biomechanical advantages and disadvantages to sumo deadlift. While I would argue the net of these factors results in an advantage for sumo deadlift, this may not be the case for everyone.
The biggest biomechanical advantage for sumo deadlift, in my opinion, comes from a reduced load on the lower back. This can be seen in how sumo deadlifters begin the lift in a more upright position compared to conventional deadlifters. They can get their hips closer to the bar and limit shear stress.
This is also backed by hard data. Our first study noted that, "The sumo group had 5-10 degrees greater vertical trunk and thigh positions."  A second study found that, "The sumo deadlift style resulted in a 10% reduction in the joint moment and 8% reduction in the load shear force at the L4/L5 level when compared with the conventional lifting style." 
With that said, getting into a sumo stance isn't for everybody. While a lot of people debate which type of deadlift is best based on their body type, the bigger concern in my mind is mobility. If you have poor mobility, it's going to be harder to get into an effective sumo deadlift stance. This stance and the muscle groups involved usually make it tougher to generate explosion off the floor, as well.
They Target Different Muscle Groups
If you just use the eyeball test, you'll see that sumo deadlifts target the quads and hips more than conventional deadlifts, which heavily engage your entire posterior chain. For this reason, some strength athletes choose to incorporate both deadlift styles into their training. Perhaps most famous of all is strongman Brian Shaw. While not a necessity, it's something to consider if you want to change up your program and try to strengthen muscles you don't typically use.
My Experience With Sumo Deadlifting
I did conventional deadlift for years (around 9 or 10 if I had to guess). After an unrelated back injury that resulted in bulging discs, I decided to give sumo deadlift a shot to take pressure off my lower back. It took a while for me to feel comfortable getting into a proper stance, and I initially struggled with technique. However, I was able to pull just under my conventional max in a couple of months.
In fact, I pulled 15lb under my highest ever conventional deadlift before I truly felt comfortable with the lift, and was soon able to set rep records at weights I struggled with on conventional. This makes sense when you consider the reduced range of motion with sumo deadlifts, and that less "work" and energy expenditure go into each rep.
Sumo vs Conventional Deadlift
So, what's the final verdict? I would say that sumo deadlift is undoubtedly easier (not the form, necessarily, but to pull heavier weights). Do I consider sumo deadlift cheating? No. I consider it a different lift altogether. I truly believe there should be different classes in powerlifting competitions, for example, to account for the general advantages sumo deadlift provides. You wouldn't have some competitors do incline bench press while others did flat bench press. They're both a type of bench press and look roughly the same, but are obviously significantly different. There's a reason sumo deadlifters who switch to conventional deadlift cannot pull as much weight.
Competition committees should also rein in form abusers (like the IPF recently did with big arch bench pressers). There are some sumo deadlifters who simply widen their stance to reduce range of motion further and make the movement easier. That's actually what I did to pull 585lb. I had no business touching that weight having just switched to a sumo stance, but was able to do it without issue.
Should you do sumo deadlift? It depends on your goals. If you're a powerlifter who can add 50lb to your total because of the lift, then why wouldn't you? Sure, you might take some flak from the fitness community at large, but it's allowed and can still be impressive. Just look at guys like Jamal Browner and Danny Grigsby (though Grigsby takes advantage of what I'd consider an excessively wide stance).
Sumo deadlift is certainly a legitimate movement to consider incorporating into your strength program, too. As you likely guessed from my experience, though, I think conventional deadlift is the way to go. There's no faking a strong conventional deadlift, and we all know it's a much manlier lift ;)