There's two problems with the 6.5 Rem mag. First is the rifle it was introduced in, the 600 series Remington. Some people called them ugly and some of us had quite afew of them. But they were a short action and bullet's had to be seated to deep to clear the magazine. Then about the only load you could get, that I know of, was the 120gr load. Then there was one more problem, it was a 6.5 and american shooter's just never took it serious. But try and get one away from someone that uses one!
In a long action rifle that could be a good cartridge except I'n not sure how easy it is to get cases for. Actually it's big advantage over the 25-06 is that went the 25-06 run's out of good hunting bullet's, the 6.5's are just getting going.
If your thinking of getting one I'd say not to. Problem is you don't reload and likely any ammo you find will be loaded to work thru the old short actions. Finding ammo may also pose a problem. Your better off with either the 25-06 or the 7mm-08. To bad, I think the 6.5 Rem should have been a great cartridge. Oh, Remington did finally chamber it in the 700 but it was to little to late for a calibre that's never been overly popular. Funny thing, most guy's that study these things think that one of the best balanced cartridges we ever had was the 256 Newton, a true 6.5/264 cal!