It's actually a great cartridge. It was designed in the early 1890's and is popular in Europe. It has been successfully used for various sized critters from varmints to moose. It was a military round for many years and can be found in surplus small ring Mausers. The original loading data is very mild but when it's built on a commercial action, ammo that is loaded to modern pressures will perform well enough to take any deer in North America.
The long 6.5mm bullets have a high sectional density and penetrate very well as well as have a high ballistic coefficient. A 140 gr VLD bullet will out perform a .308 Win with 168 gr Match bullets at 1000 yards.
In 1997 Remington standardized the .260 Remington which is a .243 Win necked up or a .308 Win/7-08 Rem necked down to 6.5mm. Ballistically it is close enough to the 6.5x55 to call them equal. The .260 Rem has a large following in High Power Silhouette matches. It offers low recoil, ample power, and can be extremely accurate.