They look more expensive than the equivalent Mikuni flat slide carb... and either one you'll probably have to tune - if you're experienced in carb tuning that's probably not a huge deal, otherwise it's many hours work.
I think the plug and play Murray's round slide VM34 kit is a good solution for most people, and represents pretty good bang for your buck. You can eke out more performance from a flat slide setup, but for street riding I don't think you'd see a huge benefit in the real-world.