Upright bars are great. It's such an easy solution for so many people who's bikes either don't fit correctly or are too aggressively positioned for the kind of riding they do. And it's relatively easy to find a setup that works given the range of options that are available today.
For mountain bikes it's easiest if you pick bars that you can use with the existing brake levers and shifter, although that's not to say you have to. The original moustache bar was made for a mountain bike after all.
By Far and away the most popular is the Soma Oxford. It's based on a classic design and offer a generous back sweep and an a small amount of rise.
I think they feel best in the riser position with the ends tilted down slightly. Some people prefer to slip them over for a slightly more aggressive, weight forward position. You can use them with revers levers or bar-cons down the road.
In the same vein the Sycip Wonder Bar and the Nitto Albatross offer similar positioning.
The Soma Moustache II and Lauterwasser bars are more forward reaching options if you want to have more hand positions but don't necessarily want to be more upright.
If you more or less like your weight distribution but you need a more ergonomic hand position the sparrow and the Clarence bars are good options.
For road bikes you can always use any of these bars with a short pull brake lever, but I prefer the feel of a moustache bar with a road brake lever. They even work well with integrated shifters!
I set these up with the tip of the brake lever in line with the bottom of the "drop" like old school road bars.
