Interesting question and I'm not convinced there is a single definitive answer for this one. I followed the threads on the 1200 for years, thought about going 4 stroke but all of the complaints including things like "heavy handling", throttle lag, poor fuel economy, clutch issues, etc. etc. kept me away.
That is what ultimately led to me ordering a 900 Ace as my entry into the world of 4-stroke machines. I bought that sled for my son to ride but I put a few miles on it as well and it is a fantastic all around trail sled. So much so that when I sold my last 600 Etec I was convinced I would go with a second 900 Ace as its replacement. My only hesitation was missing the top end of the 600 Etec. Why did I go with the 1200? Because during the no-snow fire sales two winters ago it was only a $650 upgrade for 1200 over the 900 Blizzard.
So here is my experience after putting some miles on the 1200. Best additional money I could have ever spent. The 900 Ace has excellent low end torque which is perfect for trail riding. The 1200 has more. The 900 Ace is incredibly smooth, quiet and has throttle by wire. The 1200 (MY 16' and up) also offers all of the above with the addition of a slightly meaner exhaust note that is not obnoxious at trail speeds. It just sounds better than the Ace to me. The 900 Ace has been reliable as snot. So far my 1200 has also been just as reliable. The 900 Ace has excellent acceleration but tapers off once you hit 70mph. The 1200 accelerates right to 90mph with zero effort.
So while I cannot add much regarding any of the years prior to 16', I can say for a fact that I have experienced zero of the complaints others have had about the platform in years prior. If your worried just spring for 16' or newer. You can't go wrong.