I am not a sidemount diver, but here is what I learned from the couple of experiences I've had with it, and some thoughtful reflection during subsequent trips.
It FEELS wonderful. Having the center of mass of the tanks aligned with your center of gravity leads to a feeling of freedom in the water that's simply unmatched -- it feels even better than a well-balanced single tank. It gives you stability in many positions in the water. I loved the feeling.
But my primary interest in sidemount was as a way to ease carrying heavy gear, especially up and down multiple flights of stairs, as we do in many sites in Mexico. What I found, once I started diving stages, was that it was actually EASIER to carry two cylinders on my back, than to make two trips up and down the stairs, carrying one each time. With them on my back, I could use my hands to assist my balance and to work my way up difficult steps. With the bottles carried, I lost one hand always, and often both; I'm short enough that I can't carry an Al80 by the valve without significantly bending my elbow, which means it becomes easier to hold the valve in one hand and the base in the other. (I can't do the tank on the shoulder thing because of prior clavicle fractures.)
At sites like Jackson Blue, where you can run a hand truck right up to the water's edge, it isn't an issue. But at Grand Cenote or Pet Cemetery, the logistics of running multiple bottles up and down looms large. It's the biggest reason that I never went on to convert.