The 6.5x55 may not be any better than more conventional rounds (i.e. 30-06 & 308), but it seems like it is. I've shot severals 6.5's and all were infallibly accurate, mild mannered and easy to shoot and easy to load for. They were accurate with almost every compoent bullet combination. With most rounds, you strive to find a load that shoots good. With the 6.5x55, it was more like trying to find a load that did not shoot good.
I believe the 30-06 is a superior game getter because it's bigger, heavier bullets, etc. I would not hunt elk with the 6.5 but would have no problem doing it with the 30-06.
In comparing the 6.5x55 with the .270 Win, etc., you have to al low for the fact that there are great differences in ballistics. American factory loads with the 140 grain spitzer are about 2,600 fps. I've chronographed factory Norma 139 grain bullets at 2,750. I've seen handload manual data that shows 2,800 with the 140 Grain and 3,000 with the Nosler 125 grain partition bullet.
A 6.5 mm (.264") 140 grain bullet at 2,800 is going to shoot flatter and have greater striking energy at longer ranges than a 30 caliber (.308") 150 grain bullet at the same speed. The 6.5mm (.264") 125 grain Nosler Partition at 3,000 is sufficiently close to the performance of factory .270 Win factory 130 grain loads at 2,950 that I would put such a 6.5 at parity with a factory .270. I do think the 6.5x55 140/2,800 is superior to the .308 Win 150/2,800.
On the net there is some guy with "duplex" loads that claims ,2900 with the 140 grain nosler and claims that it's superior to the 270 Win(!). "Duplex Loads": this means using two different powders in one load. I forget which powders and how much, but he posted on the web.
I like the 6.5x55. It's a "fun" caliber for me, but I quickly grew bored with it because it was so easy to make accurate loads. Somehow my "problem children" rifles always got more time, attention and MONEY than my boring rifles that always shot accurately and never gave me any problems. (Why is that?)
I don't think the 6.5x55 is necessarily any "better" than such old stand bys as the .270 and 30-06, but I do think it's better on paper than the .308 Win. There is "something" about the 6.5x55. I fiiddled around with it in milsurp rifles, but I know people who were so taken with it that they obtained commercial sporting rifles such as the now discontinued M-70 Featherweight and Howa 1500 in that caliber and some people have had existing sporters rebarreled to the 6.5x55. I was at the range one day and there were three people with sporters in 6.5x55. They were together, kind of like a club. One of them bought a Swedish mil-surp, tried it, loved it, rebarreled a regular sporter, hunted with it, loved it, shared it with his hunting buddies and they were bitten by the 6.5 bug.
All love the accuracy and low recoil. They say the 6.5x55 has killing power disproportionate to its ballistics. I think that's probably a subjective kind of conclusion, but I've heard it from many different people. I think it is because the 6.5x55 is easy to shoot and easy to hit with and because a hit with the 6.5 will kill better than a miss from the more powerful 7mm Rem Mag.