I have used the RAMLINE and DPMS synthetic stocks on my Garand. The RAMLINE was a drop in fit, but the DPMS took alot of fitting. I have been pleased with both. The RAMLINE stock did "seem" to make my Garand a bit more accurate, but since I have done other things to it, I cannot claim that the stock alone made it more accurate.
The DPMS stock on the other hand did improve the groups. The DPMS stock is a very tight fit and has remained so after several range sessions where 200 to 300 rds were fired each time.
The advantage to synthetic stocks for me is in the durability over wooden stocks. I had a very nice wooden stock on my Garand. I took my time and refinished it and it looked great...for about 15 minutes. Then as I was going to answer the phone, I hooked my foot on the buttstock as the rifle was leaning up and skeetered it across the concrete floor. I replaced that stock with a synthetic and now I don't worry about nicks, dings, etc. A little Krylon fixes things up just fine now.