Try the Tartan Owners web site. Go to the Classics Association and look at the reveiws under "Resources".
I just purchased a 1973 Tartan 34C Yawl w/wheel & small diesel in Jan. and all I can tell you is that I love the boat. She was in good, but original condition. We have be restoring the boat for a while now. (splashed it 1 Sept.) Concerns with this boat are same as any boat of this era. Sailed a family race on Labor Day in light air and... she's no J-22. However, in 10+ knots of air she's quick. I would have to dis agree with Jeff on handelig chop. She seems to cut thru the confused seas on lake Michigan much better than many other boats that tend to "slap" down on waves. I also disagree with the assessment of it being a wet boat. During delivery we were sailing in 35 knots of breeze and 6 - 8 ft. seas and the boat never took a wave over the bow.
With keel/CB she is a bit tender early, but seems to firm-up well. Many contend the 34C sails best flat on her feet, however, from my experiences so far she seems to gain speed with the extra water line gained with a slight heel. Space below is a little tight compare to new boats of this size. The main saloon is smaller than what I had on my previous 30 footer. The smaller cabin does translate to wide side decks and a comfortable deck and cockpit layout. Wheel does get in the way when moving from cockpit to cabin as it is located at the forward end of cockpit, but this allows for a lot of space for guests aft of the wheel.
All in all I think it is a great boat with a lot of info available thru owners group. Probably the most popular of S&S designed boats with noticible "Intrepid" inspired design below the waterline.