Oh, I dunno...
I've read a couple articles on the 750 and the 500 twins in Motorcycle Classics. They gave conditional positive reviews to both. The early 500s earned a bad rep early on for genuine issues - balancer chains that stretched out of sync, making vibration worse than no balancer at all. They (early 500s) also overheated the heads, leading to cracking exhaust valve seats. Then, too, they weren't exactly powerhouses ever.
By the end of the run (late 70s), the problems were pretty much fixed, but, give a dog a bad name and you might as well shoot him.
The 750s never really suffered like the 500s, but, they never really were very popular either. Gotta remember the market dynamics of the time: twins were on the way out, and the UJM (an air-cooled inline four) was on the way in. The 650 was the exception as it soldiered on...
Here's a guy who wants a '78 XS500:
http://orlando.craigslist.org/mcy/709199746.htmlI had a 750 triple. It had the "famous" Yamaha problem of jumping out of 2nd gear, but it had 35K miles on it, so SOMEone must've liked riding it...
But then, I'm a real sucker for the fringe, or Weird-Alice machines! I can't EVER see myself owning a Yammie 650 twin for that very reason. Too common.