DS is a strong, reliable motor in a pretty tough package. They are reliable and wear well.
The knock on the DS was for a long time, and still is to a lesser degree after the changes were carried over from the "X" line, that it's too big for tight trail riding. The size, the weight, the physical dimensions, etc. These same attributes are what makes it such a joy to ride in the sand and wide open spaces.
Now I wouldn't knock someone for getting the DS, but a lot has changed in the atv world since they came out. The shaky handling of the 660 was replaced by the 700, and with the newer shocks it's right back up there even if the travel is a bit shorter. The 700's engine seems to be able to handle the ridiculous power outputs the DS guys have been getting in the past, not that it's all that relevant to a trail rider. The DS is a hard sell up against the 700, it fares much better against the 660 which was designed and released around the same time and ended almost the same time. If you are interested in one the later X models are the best, I think they had them from 03-06 but I'm not sure on that. It should also be noted that Can-am (bombardier) discontinued the DS650 back in 2006, the year the 700 first came out.
In short, I suggest you take a look at a DS but ultimately I think you'll be happier with and prefer the 700.
Oh and whatever you do don't go to www.can-am.com
.... that's not the can-am website..... it's www.can-am.brp.com